Why my computer is slow?

well thats a broad question unless any technichal specifications are their......on the other hand , if try to answer the question without any details....i would suggest either or combination of these are the culprits:
1. slow processor
2. lesser ram as coompared to work you try to do on ur comp(get extra RAM)
3. heavy antivirus running like norton, mcafee,etc(if so try installing AVG free version)
4. bad sectors on hard disk(many cases can arise so contact ur vendor) or highly fragmented hard disk(try defragmenting the disk using windows in built defragmentor, it would be better if you can download a defragmentor software...there are alot on www.download.com)
5. have not formatted ur system for ages..(its better o format atleast ur Os partition time to time)

(Now comes the reasons that are destructive:)
6. viruses have taken over ur system( so most of the ram is eaten by'em.solution: try formatting whole of the system)
7. trojans,adwares,spywares,etc...(solution… try formatting whole of the system)

and their could be many more reasons i could point out but may not help you unless you specify details.....(how can a doctor cure a patient if patient does not specify the details)