Jinky, thankfully there is a way.
(First, you might want to write down the version number of your MSN 5.0, such as 5.0.0544, which can be found in Help -> About MSN Messenger)
Download: Hex Workshop from
http://www.bpsoft.com/.
Be sure to have MSN 5 exited before you use Hex Workshop.
Open MSN 5'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 5. (That also include the build number, i.e 5.0.0544)
In the Replace box, type 6.2.0544. Click OK. Click Replace All. Click OK. Exit Hex Workshop. Open MSN Messenger 5. You should now be able to sign in.
I hope this helps.