Well, that's not a software change, that's hardware.

You have to open up your computer, take out the hard drive, and switch the current primary to the "slave" setting and the other hard drive to primary. Your computer then will attempt to boot from the primary drive.

For more on that, google for 'how to change a hard drive from slave to primary'.

Well, let's start off with the basic questions:

1. Is your game a pirated copy?

2. Have you modified your game content in any way? (Aside from addons)

3. Do you have virus scanning software?


Before you answer, do the following:

Scan your computer for malware/viruses.
Reinstall your Garry's Mod by deleting the "Steam\SteamApps\<Steamname>\garrysmod" folder.

If that doesn't fix the problem, reply by answering the above questions.