So, you haven't had your pc long enough for the trial period to expire and you're infected with a browser hijacker already?
Go here and download AdAware:
Go here and download Spybot Search & Destroy:
Get the latest updates for both pieces of software, and thoroughly scan your system with them.
Remove every single piece of spyware (critical or otherwise) that those two programs find.
Next, click here to download CWShredder:
Run it (there is no installation) to clean your system of any CoolWebSearch hijackers.
After you're done with all of that, let us know if you can activate your Windows XP. Then, immediately get all of the latest Windows updates to help keep you from getting infected again.
After all of that, consider using Mozilla Firefox as your primary web browser. That will keep you from getting just about any kind of spyware infection that exists. You can get Firefox here:
If you still can't get Windows activated because of a browser hijacker, post here. I have some other tricks up my sleeve.