Remote Desktop has stopped working

Thanks in advance for your help.
We have a small hardwired LAN network with four similar Dell PCs running XP Pro SP3 and a server running Windows SBS server 2003. Remote desktop worked with all PCs and server but suddenly stopped on one PC.
When I try to connect remotely from home, or even from another PC on the same domain I get no error message, just a quick flicker and back to the initial dialogue box.
The 2Wire router configuration hasn't been changed and the port settings appear to be all correct.
There was a Windows update which appeared to switch on the Windows firewall, but switching off makes no difference.
Remote Destktop and Remote Assistance are both ticked in PC properties and my username has access as part of the domain administrators.

I've rebooted PCs and Router and tried un-ticking and re-ticking the right boxes, but no progress. Help!
Thanks again

 

Many thanks for your reply.

Re-registered RD successfully, but same problem.

No error message when running telnet from a remote computer.

I suppose re-installing Windows may be the answer, but I'd rather avoid that if possible?

Thanks again.

 

I suppose system restore might be an option to try?

 

And then I find that System Restore looks as though it switched on, but actually needs activating!

Just done a full rebuild - problem solved.

Thanks for your help.