2) Viruses

3) To many programs open (Also too many background items open)

4) HardDrive space

You can speed up your boot time by clearing your browser CACHE by going to Start> Run> type C:\WINDOWS\Prefetch and delete everything in that folder. (This will not harm your PC)

Speed up internet explorer a little by (This is for IE 7.0) Go to Tools> Internet Options> Delete> Delete all (This will delete your browser cookies & saved info that was entered in forms (Such as remembered passwords or stuff that you typed on Google)

You can also delete uneccesary items from add/ remove programs that you know you don’t need. ( Don’t delete neccesary stuff such as Word Proccessors { like MS Word} or don’t delete flash player plugins. Just delete any thing that you don’t use that takes up 30.0 MB or more space on your harddrive

Wall papers large photos can also lag your computer especially on boot up, If I was you id use the default blue background.

Another thing that you can do to speed up your computer is buy more RAM. If you have 2 RAM slots them buy 2 Gigabytes of RAM or if you hae 4 slots then buy 4 Gigs (Even if you have four slots, 2 GIG should be enough for normal use, gamers should get 4 gigs)

You can back up your harddrive and then install its contents onto a bigger harddrive (Atleast 150GB) and you will have adequate space to download programs (250GB + for gamers)

Download SpyBot Search & Destroy to get rid of any malware, adware, or viruses lurking on your PC.

Disable uneccesary programs on start up using MSCONFIG in the Run program.

You should also upgrade your CPU (Central Processing Unit) or add a second one (Core 2 Duo is a good one if you don’t already own one) If you dont know what you are doing then have a Pro install one for you. When it comes to buying proccessors, it can becocme pricey.

Another way you can speed up your PC is to delete any bloatware that by default – comes with Windows. Bloatware is usually the games that comes with it, Trial software ECT.

~Kweezy157
References :
View this link to see 23 tips on how to speed up Windows eXPerience (Windows XP) assuming thats the OS that you are using.

Congrats to me this is the 200th answer ive given since ive started Y!A =D

Malware, Spyware, and all sorts of Viruses need to be cleaned out of the system.

You could start cleaning out your system by going to Start>Run and type in %temp%

Delete all those files!
References :

Spyware..

Virus…
References :

what are the TOP REASONS for a SLOW computer?