Hi,

I have a HP a1310y case with 300W psu that I rebuilt with an ACER AST690/apfh motherboard & a Pentium4 672 HT\VT CPU, 3.8 GHz, PC 6400 800 MHz. I have 2 SATA diskdrives 150 GB & 40 GB. To get the machine running, I used the 2X1GB DDR2 RAM PC 5300 667 MHz RAM from the old motherboard. Iknow, it'san lod system with used parts
I run with Windows 7 Pro. SP1. As-is,the system is fast enough for me and I have the Virtual Technology to run my Win XP files,but it hangs up and/or runs slow at odd times. Also, if I do much more than surf the internet, the heat sink fan goes to high speed. The CPU runs 38C at idle & 57-58C under load (fan 5800 RPM)
My question is, does the mismatch of RAM speed and CPU speed cause the hangup/slow down and heating of the CPU?

First thoughts are to check the AV schedule, and perhaps the active virus checking: when using MSE for example, it can take 20 mins for the AV system check on startup to settle down.
Have you observed system loadings in Task Manager?