As you are probably aware, you have 30 days in which to activate Windows XP. There are ways to prolong the activation period indefinitely that we won't go into as that is probably illegal.
You can restore your operating system to any time that a restore point was created, automatically or manually. Naturally, it makes sense to only restore to a point where you know the comp was running good. No re-activation is required after a system restore.
I suspect that your problem may be more than simply restoring, but it is worth a try.