Computer help forum: Laptop Keeps Freezing 5--10 Minutes After Start-Up

I own an Acer Extensa 5630 notebook which runs on Vista Home Premium. I have been using it for almost 2 years with no problem until a few days ago it starts freezing up around a few minutes after the computer starts up. Here is what happens... So the computer starts-up normally just like a regular computer. However, after around 3-5 minutes of normal usage using low-demand programs (IE or Excel or just any programs) the computer will stop responding and then on top of the program will show (Not Responding). The cursor can still move but the keyboard is not responsive. I cannot bring up the task manager so I have to hard reboot the computer by pressing the power button. So basically my computer is unusable right now cuase it will freeze up like theres a timer around 3-5 minutes after it started-up. I cannot even do any scans or system restore becuase it will freeze up before I can reach any programs. But the funny thing is the computer works perfectly fine like a normal computer for the brief 5 minutes before it decides to freeze up again. I did not install any new programs and I am very certain it is not due to virus/malware. I am not able to reformat my notebook becuase there are alot of valuable information in my hardrive. Nor do I have an vista installation disc to do repair on my Windows.

Here are some specs:
Intel® Core™2 Duo Processor T5800
(2MB L2 cache, 2.0GHz, 800MHz FSB
2GB DDR2 667 SDRAM
160GB HD

please tell me if you guys need more information, any help is appreciated, thanx!!!