It is a Dimension 8100 desktop. I have attempted to defrag but it froze during that. I also attempted to start in safe mode but for some reason the keyboard wouldn't allow me to push the up or down keys so it would just continue to start normally. Please let me know what you think. Thanks!

u have to get a error message if the system is freezing in the middle of something there might be software conflicts. do the following Go to Start, Run, type in "msconfig", no (""), select OK. Now click on the StartUp tab. Uncheck everything that you don't need running in the background. keep systray and explorer and scan registry checked.
then run scandisk. then run defrag. make sure you stop your screen saver from running

 

if this dont help run spybot search and destroy. also run a spyware program,antivirus program if all else fails your power supply could be going causing heat inside the case causing the freezing