If the speed was normal after reinstall until a certain program or update was installed it might be the culprit.
This can be determined by reinstalling following these same steps to get a perfect baseline Clean Reinstall - Factory OEM Windows 7.
After installing the necessary LAN driver
to get online if it's not provided, then run all rounds of Important and Optional Updates after enabling Automatically deliver drivers via Windows Update (Step 3)
Then run for day or two before installing any programs - besides Flash, Java, Adobe Reader. Instead spend the time setting up your desktop, copying in files.
Once performance is confirmed with only Updates, start installing programs slowly over time with enough time in between each to test performance. This should isolate the exact Update or program that's causing the problem, you can System Restore to get before it and post back here for help resolving it.