Something has changed on your computer. What? Who knows!
You didn't mention if this computer was dedicated to recording or if it was used for internet, email, etc. If it is used for internet, I'd say that this is the time to switch to a dual boot system with internet disabled on the recording operating system.
I find that it's faster to simply format and reinstall than it is to troubleshoot. After you have reinstalled your system and have it working as it should back it up like this:
Create A Perfect Backup Of Your Windows Hard Drive
If you really want to troubleshooting it check out these:
Recording Computer Troubleshooting: Is Your Windows Operating System Clean? Home Recording Computers: Clicks, Pops, and Latency
Brandon