You will still be able to use XP as before. At least till it becomes totally obsolete. The only thing that will happen is MS will no longer provide updates fpor XP. This includes security patches. Thus it will become ever more vulnerable to malware attacks. Your security protection will gradually become less effective as companies optimize their security suites for more modern OSs.
It might still be a good idea to grab a copy of Win7 while you still can. Just in case. Win8 is an even bigger problem that Vista was. At least in terms of sales. MS will eventually come out with 9. Which will possibly be as good as 7 was after the Vista fiasco. That may come out as soon as a couple of years but may take longer. Who knows?
Win7 Pro 64 bit is a great OS, faster and more secure than XP and it can do even more. You should determine if 7 will even run on your old computer before buying it, however. You may have no choice but to buy a newer computer.
I hope this helps, Paula