Are you working on a mod for "Half-Life 2" or some other Source-engine game? When using "Hammer," are some of your entities not properly showing up, or do you get an error about a bad VProject environment variable? Does HLMV just show black instead of your model? Does VConfig take a long time to set everything after you hit "OK?"
I was having the same problems today, and I have a cause and a solution for you.
The issue is a setting in Windows Explorer, under Tools → Folder Options... → View → Advanced Settings. There is a setting there called "Launch folder windows in seperate process." This setting increases the stability of windows, but can cause problems with multiple (and occasionally conflicting) copies of environment variables hanging around.
You have two options to fix the above problem. You can either:
1. Disable the "Launch folder windows in seperate process" option, reboot your computer, then run VConfig again; or
2. Run VConfig. Click "Edit..." Change one character in the path and immediately change it back. Click "OK." Click "OK." Run VConfig again. Click "Edit..." Change one character in the path and immediately change it back. Click "OK." Click "OK." Close all Explorer windows, including Internet Explorer. Open Explorer and run VConfig again. Click "Edit..." Click "OK." Click "OK."
If you disable the "Seperate Process" option, VConfig will run faster, but Explorer will be less stable. However, the Source SDK Tools will work.
If you use option 2, VConfig will still run slow as molasses, but the Source SDK Tools will work. However, if you have to change to a different game in VConfig, you will have to repeat the steps in option 2 all over again.