Random Processes Take 100% CPU, Slow Computer

I just bought a compaq presario laptop a month ago, and now it's having problems. Every time I turn it on for a few hours, it starts to run at close to 100% CPU constantly, even when I'm not doing anything. I check the running processes, no single process seems to be doing the thing. It would be Mozilla using 80% of the CPU power while tskmgr.exe uses 20% one second, then next second it would be explorer.exe using 50% of the CPU power while svchost.exe using 50% of the CPU power. Sometimes, the processes would show that System Idle Process is at 95%, while the performances tab would indicate that 100% CPU power is being used, which is absolutely confounding to me. End tasking processes don't work, it'll just be different processes or some imaginery process taking 100% of the CPU power.