It's not just a reload...It's an ADVENTURE
From the fact that you bought your PC on Craigslist, I'll assume that you do not want to spend gobs of cash, and do not lust after the latest whizz-bang features. Even so, you may find buying a new PC to be the most cost-effective solution, since many name-brand manufacturers now offer PCs at prices well under $1000...and in today's economy, they really want to sell them, which gives you some leverage in making a deal.
If you really can't see buying a new PC, then here are a few typical situations you may face:
1. Your best resolution is to find an experienced computer builder (hereafter called "hired geek"), and let that person do the work I'll outline below. Tell that person that you don't want any "upgrades": You just want this PC to work. If you go to one of the big-box retail computer stores for service, they will try to sell you hundreds of dollars in new parts, and if you can't say NO, you may end up paying more to "repair" your old computer than you'd spend on a new PC.
2. Make a list (on paper, not saved on the computer) of the "programs" (word processing, email, and so forth) you use on your current PC. Show the list to your Hired Geek and have him/her give you an estimate of how much you'll have to spend to replace those programs. Don't forget to include antivirus, firewall, and malware-scanning software (I will discuss these below). Bear in mind that there are great free alternatives to many programs.
3. Since your PC is at least a year old, buying Vista for it is a gamble, so stick with XP. Windows XP installation CDs are fairly cheap: Just do a Google search for "Windows XP SP1" and you'll see a few prices under $150. Note: You'll have to choose between the "Home" and "Pro" versions. If you really plan to keep your old PC for a few years, then you do not need to pay for the "retail" version, which runs about $240-$260: The cheaper "OEM" and "DSP" versions are fine for your purpose, but you should avoid the "student" version and the "upgrade" version. I'll have more to say about "SP1" below. For now, just assume that SP1 (not SP2 or SP3) is your ticket to ride.
4. Assuming that you did not get any software or driver CDs with the secondhand PC:
A. Check the Device Manager BEFORE you wipe off the old OS, to get the full name (Brand and model number) for your Display Adapter, Network Adapter, Monitor, and Soundcard. Each of these requires a specific driver to work optimally -- or work at all.
B. If you have a "brand-name" PC (Dell, Gateway, H-P, Sony, etc.), its manufacturer may have had the video or sound card custom-built -- thus requiring a similarly custom-written driver -- so that even though the card still carries the name "GeForce", "Radeon", or "SoundBlaster", it may not work reliably with a driver downloaded from the Nvidia, ATI, or Creative website. Some manufacturers websites can tell you what video, audio, and other "daughter cards" were shipped with the PC, if you can give them the system serial number (check the PC case for it). In many cases, you can then download the necessary drivers from that manufacturer's website.
C. But what do you do if your Craigslist seller swapped in some cheaper parts before selling the PC, if there's no manufacturer's serial number, if you can't open Device Manager, or if you have already wiped the hard drive clean? Then you (or someone who builds PCs) will have to open the PC case and autopsy the innards. If you are not ready to buy another PC, and have no experience working inside the case, go no further: Find that Hired Geek and pay him/her a few bucks to do the autopsy and download the drivers.
D. In many cases you will also need something called a "chipset driver" for the motherboard itself, and as with the sound/video/network cards you will have to know who built the motherboard, and what "revision" it is. While the hired geek is chasing that down, s/he should also verify that your motherboard BIOS is the latest stable version.
5. Odds and ends:
A. Why buy a Windows XP-SP1 CD when you can get the later "improved" SP2 and SP3 versions? First (assuming that you are cost-conscious), the SP1 CDs are a bit cheaper than the SP2 or -3 versions. Second the SP2 version is in short supply: You may only be able get SP3, even when you have ordered SP2. Third, SP3 has had "issues" with some printers, scanners and other devices, and even with some AMD processors. Most of SP3's problems have been fixed, but the fixes must be downloaded from Microsoft's website, and you may have difficulty doing that if your PC is having SP3-related fits. Once you have Windows XP-SP1 installed, you (or your Hired Geek) can download the SP2 upgrade, install it, and decide whether you want to go to SP3 later.
B. VITALLY IMPORTANT: If your PC has been connected to an "always on" internet connection (cable modem or DSL), you MUST disconnect the PC from the modem before you install a fresh copy of XP. You should not reconnect to the internet until you have installed a "firewall" and an antivirus program (ask your Hired Geek, or do a Google search). Without a firewall, it is possible for a certain breed of internet sociopath to find and "hack" your unprotected computer, and in theory this can happen in a matter of minutes from the time you expose your system by connecting it to the modem. Because of this, your antivirus, firewall, and malware software should be downladed and burned on a CD before you reload...or your Hired Geek can do it via his/her own system. Once those security measures are installe, you can put your PC on the internet, "activate" your XP installation, and download updates for your antivirus and malware scanners.
For geeks like me, all this research and preparation is part of the fun -- part of the pride we take in a job well done -- but for non-geeks and folks with a desire to "just make it work, d#%n the fuss", resurrecting a dying PC can be a big mountain to climb. I don't want to scare anybody off the attempt, because it is a great education, but there are alternatives, and I hope I've made a fair case for them.