Here's what I would do:

Make sure your disk drive is not failing, even if it is new: a basic chkdsk cmd should do (It not unusual to have a few back blocks) or you use something like seatools.

Load MicroSoft Security Essentials (It's Free) and scan your computer for viruses.
Load SpyBot Search and Destroy (by Donation - Free) to clean out the adware and the like.

There are two tools I use to make sure nothing is loading that I don't want loading on my computer.

Autoruns and ProcessXP. (Both Free)

For keeping your computer clean from trash and unneeded cache, tmp files and the like, I use CCleaner. (by Donation - Free)

If you computer is still being hijacked after the above, then google hijackthis and get some help from the forums. Else make sure your hard-drive is not failing.

A bit of prudence: Hard drives are so cheap that you can just keep a complete image of your system on another hard drive. Hard drive Manufactures have free cloning utilities for upgrading your hard drive and that make imaging very easy.

HTH