Recently, I realised I only had like 8 GB left on my C:/. out of 150. So I decided to move Steam to my D:/ which had 100 GB free anyway. I backed up my steamapps before installing Steam to D, and then moved it in after installing Steam again to D:/. Now I have alot of space on both, which is great, but when I start Hammer, It says that it couldn't find the gameinfo.txt, obviously because it searches in the old path. (It states the path its trying to find it in).

Anyone know a solution?

Set your "sourcesdk" environment variable to the correct path.
System properties - Advanced tab - Environment variables...