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
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