There are a couple of things you could try
First of all, open Internet Explorer, go to the tools menu, then options (If you cannot see the tools menu you're probably running the newest version of Internet Explorer, so press alt and the toolbar will appear)
Under the general tab that pops up you should see delete history delete cookies and delete temporary internet files buttons (Newest I.E its 1 button that take you to a menu where you pick which things to delete, can't remember what its titled), delete all of these things, also check where your hompage is, I think its still in the "General" tab, point it to google incase its the homepage thats causing the problem.
A 2nd thing you could do is roll back to the earliest System Restore point you have (not something i've done often enough to know how long it keeps the points)
To do the system restore (on Windows XP):
1. Click Start, point to All Programs, point to Accessories, point to System Tools, and then click System Restore.
2. On the Welcome screen, click Restore my computer to an earlier time, and then click Next.
3. On the Select a Restore Point page, select the date from the calendar that shows the point you'd like to restore to
4. On the Confirm Restore Point Selection page, verify that the correct restore point is chosen, and then close any open programs.
5. Click Next if you are ready to proceed or click Back to change the restore point.
6. The computer will shut down automatically and reboot. On reboot, you'll see the Restoration Complete page, and then click OK.
Third and most drastic solution is the reinstall the operating system.
**WARNING** Doing this will delete any files you already have on the PC, make sure you back anything important up or it will be lost forever!
1)When you bought the computer you should have gotten a "Recovery Disk", If Windows came preinstalled on your PC but the manufacturer didn't provide a full Windows XP CD, contact the company and ask for one (you did pay for it, after all). In most cases, you'll get one for free, no questions asked.
2)Reboot the PC
3) While starting up press del, this should take you to a blue screen with a few options, If you see the windows logo you're too late. If del doesn't work try using F2, different PCs have different ways to get to this screen
4)After being confronted by a rather scary looking blue screen with many options, look for something along the lines of "boot order", it may well be inside "Advanced BIOS features"
5) Change the first boot device to CD if it is not already the first.
6)Insert the Recovery disk into the CD drive, Save and exit the BIOS (blue screen), this will restart the PC
7) Now the system will boot from the cd, just follow the on screen instructions, and you'll have a brand new install of your operating system. Its a pain to reinstall every program you had before, but if its either that or have a PC that doesn't work... well... I think it would be worth it somehow.
8) The very first thing to do on any new PC is get anti-virus software onto it, if you have it on a disk, just install it like before, If not i'd recommend downloading AVG for free until you get something better, I know many people that just use the free AVG scanner, it's not the best, but it IS free.
9)Breathe a sigh of relief because its all over :)
Hope this helped.