My computer was doing the same shady stuff, and my dad (a computer program, so the freeware is the legit, non-virus laden stuff) sent me three links to sort it out:
Treesize Free - lets you know the size of the contents of a directory http://www.jam-software.com/freeware/index.shtml This is most useful for just seeing where everything is and what it's doing.
cCleaner - remove unnecessary files from your computer http://www.ccleaner.com/
Process Explorer - shows all the programs and services that are running http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx
I think this last one might be the most useful to you since if there's some creeper program messing with your computer it'll show up on here.
Hope any of those help. Computers being messed up for absolutely no discernible reason is never fun
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Treesize Free - lets you know the size of the contents of a directory
http://www.jam-software.com/freeware/index.shtml
This is most useful for just seeing where everything is and what it's doing.
cCleaner - remove unnecessary files from your computer
http://www.ccleaner.com/
Process Explorer - shows all the programs and services that are running
http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx
I think this last one might be the most useful to you since if there's some creeper program messing with your computer it'll show up on here.
Hope any of those help. Computers being messed up for absolutely no discernible reason is never fun