For troubleshooting purposes could you try another version of Vegas Pro as in an earlier build if you have the latest of Pro 11.
Your HWMonitor and Speedfan screenshots are showing bad readings for the +5V rail, this can affect the performance of the GPU among other things and needs to be checked out, see table below;
Please restart your computer and check the temperatures/voltages in the BIOS, no screenshot is required just make a note of the temperatures and the +3.3V, +5V and the +12V and post them with your next reply.
NB: BIOS voltage readings.
The readings are not conclusive in the BIOS as the computer is under the least amount of load, if they are higher or lower than what they should be though it does suggest a PSU problem.
Use msconfig to determine what is causing the problem
These are good tutorials on using msconfig in XP, Vista or Windows 7:
How to use msconfig in Windows XP
How to use msconfig in Windows Vista
How to use msconfig in Windows 7 and Windows 8
Click on Start then Run, type msconfig and press Enter.
Click on the Startup tab (for Windows 8, the Startup tab has a link to open Task Manager/Startup tab. Use that.), record what is currently starting then click the Disable Allbutton.
Reboot and see if it runs better.
If yes then use msconfig to enable several items at a time till you find the culprit.
If no, start msconfig and click on the Services tab.
Check the Hide All Microsoft Services box, record what is currently starting then click the Disable All button.
Again, do a regular boot, see if it runs normal.
If yes then use msconfig to enable services till you find the culprit.
Once you've found the culprit, uninstall it or find out how to eliminate it from your system. Simply disabling it in msconfig is a temporary fix at best.
Enable everything else you disabled.