You're just asking for somebody to say "install Linux", aren't you? I mean, you left that so wide open!
But seriously, how old is the Windows install, and is there a lot of crapware? And, is it an OEM install?
If it's an OEM version that came with crap intalled, try:
http://www.pcdecrapifier.com/downloadIf it's just old, run "msconfig", go to the "startup" tab (or whatever it's called), and disable pretty much anything unless it sounds important.
If all else fails, re-install Windows.