I had a Q6600, I can tell you, as GhostGT did, under Intel's Speed Stepping and Enhanced Halt State and similar instructions that the multiplier goes down to 6 and the end result is 1600MHz. This only happens when the OS sends an "idle" signal. This is normal.
However, slow performance normally results in the processor or RAM so lets not rule out the passive parts in the computer.
Can you get a screenshot of all the tabs on CPU-Z for me?
As others have suggested, also download HWMonitor
or a similar temperature monitoring program.
Did the tech take away the 300w you had? It would be a good idea to check that PSU in the system if they didn't take it. Since you say the system was fine before they had it
I would also strongly believe the Harddrive is not working properly. They are VERY sensitive and too many G's on impact could have thrown the actuator off or messed up the rotation of the platters. Download HDTune
and run a simple bench and screenshot it so we can determine if the speeds are normal. Also run a *quick* error test to check for bad blocks/sectors.
Go to Memtest86.com and under the Free Download section, click on MemTest86 3.4a if you want to run a test from windows for a fairly accurate test or download MemTest86 3.5 as a bootable ISO and burn it to a disk. Remember to set your CD drive to boot first.
We'll move from there if we don't find any suspicious faults.