there was a handful of things i used to do "as standard" when i was an engineer doing this sorta stuff almost every day.
first off, run antivirus. i never ran the antivirus that was installed. i always booted into safe mode and ran an isolated application there. this eliminated the chance of any potentially installed virus from actually running, and it'd be able to delete it.
next i'd run adaware, also in safe mode. for the same reasons.
then i booted back into normal more and ran disk cleanup. got rid of any temporary files that accumulate over time. never "compress old files". this can sometimes make matters worse, as you're then giving the cpu more burden of uncompressing these files before it can read them.
then run disk defragmenter. the reason i did this last was because i don't wanna defrag crap i'm gonna delete. so i delete it all first.
it's also worth looking through your add/remove programs to see if there's anything in there you don't use anymore, or have never used, or don't recognise the name. certain applications have things such as "quick launchers", in that it runs a small little program on bootup, thus initialising the main application so it's quicker to launch. however if you never use these applications, then there's no point.
this is the antivirus program i ran in safe mode. there's 3 things to download:
http://www.trendmicro.com/ftp/products/tsc/sysclean.com <-- the main application
http://www.trendmicro.com/download/viruspattern.asp <-- virus definitions file
http://www.trendmicro.com/download/spywarepattern.asp <-- spyware definitions file
download and store these all in a directory of their own. on the 2 web pages, download the latest "official" lpt (virus) and tma (spyware) .zip files and extract them into the same folder. then boot into safe mode, navigate to that directory, and run the sysclean.com file. it'll extract a bunch more files, and run itself. when it's launched, just hit "start" and let it do the work. depending on the age of the machine, this can take anywhere from 30 mins to 2-3 hours. be patient!
hope this helps.
