Yes, there is a way to get MSN Messenger 6.0 working again.
Download:
Hex Workshop
Be sure to have MSN 6 exited before you use Hex Workshop.
Open MSN 6's msnmsgr.exe in Hex Workshop.
Click the Edit menu, and click Replace.
Click the Type arrow, and select Text String.
In the Find box, type the version of your MSN 6. (That also include the build number, i.e 6.0.0602)
In the Replace box, type 7.0.0813. Click OK. Click Replace All. Click OK. Exit Hex Workshop. Open MSN Messenger 6. You should now be able to sign in.