Most likely your computer came with a "Restore CD", insert the restore cd, then boot up your computer, and follow the on-screen instructions. If your computer didnt come with a recovery cd, or you upgraded to a newer version, then you might have the original windows cd, in that case, boot off of the cd, and select the partition that you want to install it on (Only on XP).
If you have none of these cd's, then i am afraid you cant restore your computer. Although you can do some things to try and make it faster. Download Ad-Aware, SpyBot, SpySweeper (Adware/spyware program) and get an antivirus program and run them to see if thats slowing you down. Also, you may have a lot of unnecessary startup programs. Click Start>Run and type "msconfig", then select startup, and de-select all the programs you dont need to have running.
Hope that helped.