Reinstall XNA 3.1 after XNA 4.0 Installation

It told me about "cant find the necessary files/references".something like that.

SO I instantly diagnosed our patient: "The Windows Phone 7 Developer Tools somehow corrupted older version of XNA"

To solve this problem:  please reinstall XNA 3.1 and it will be working nicely again.It wont break XNA 4.0 files.dont worry about it :)

XNA 3.1 is Here!

Next Recommended Reading Breaking Changes in XNA 4.0