MSN Messenger for Macs

[Lotus]

Ive just switched from PC to Mac, and am now using MSN Messenger for Macs - but the file send/receive option is just not working when I try to send/receive files to/from PC users. Mac to Mac it works fine, but PC-Mac just does not want to happen. Ive added MSN Messenger to the firewall and all the traffic through the 6891-6900 is being let though. When I first activated this, it seemed to work - however, my joy was very short lived. By next day, it was all not-working again, and Im getting ever so slightly aggrevated by this :)
If anyone has any suggestions, I would be most grateful

[WoWo]

Don't have any advice other than that I have been able to transfer files from PC to Mac with no problems.