I use mirc and i recently got this error and until this moment i couldt get what was causing this.
But when i looked at my system log i found the following message:
"TCP/IP has reached the security limit"

after doing a google search i found this website:

where you can replace a file that removes the tcp/ip limit on a sp2 system by replacing a file in your system.

i dont know if this is finaly a fix for this annoying problem but for all of you xp sp2 users try it and please report if this fix works.