In the default location:
"C:\Users\[UserName]\Documents\Visual Studio 2010\Projects"
or:
"C:\Development\"
or:
"D:\Development\"
or...
I put them on my "Data" drive, which would be D in your case. Then, if I need to back up my machine, I can easily backup just the data and not the program files. Separately, your code should be in source control as well.