If you had a hardware problem "sluggishness" after a few days wouldn't be the result, you'd just crash, sooner rather than later, or you'd notice artifacts in 3D games, or corruption in the hard drive. (Although if a P4 overheated, it would reduce its clock speed and seem to be running slow.)
It's easy to determine if heat is a problem. Take the side off the case and aim a small household fan into it with a decent airflow speed (box fans do nicely but even a small desk fan will work if it's on high).
It sounds like maybe you've got something with a memory leak running, or perhaps some malware. You can look at Task Manager and see if anything in particular is sucking up CPU time or memory. There are some bugs which result in the service host process running at 100%, which might be occurring if a particular thing happens every few days, like a time sync.
Does the problem go away if you reboot? If so, then it's software, since nothing in the hardware would change drastically during that time.