I have to constantly power off my computer because it freezed up and that's all. I've checked the memory diagnostic, defragmented, removed programs I don't use, did a disc check, you name it. I've tried it. I'm at a loss.... What can I do about this problem?

Might see event logs to start. Set bios to failsafe or default settings maybe. (better know the original settings) Basically it comes down to two issues. One is software and the other is hardware. Software may or may not be good on your computer. Event log might show some issues. Might have to setup a dump log if system can report. If hardware it might be hard to track down. Anything from temp to voltages to bad/esd damaged parts or oscillators off or firmware not correct and more. If it did work good at one time might end up doing clean install and then monitor any changes until fails. "Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10

sounds like the heating fan failures, or maybe another silent batch virus, but i think its the fans personally. if they fail to work your system cannot cool down, many people have that problem, try checking your fans and reducing power taken with any accessories not needed