It's almost certainly a trojan or virus of some sort, I can't think of any legitimate reason why a script or program would ever do that. The first step would be to try out more Anti-virus software - the variation between what different AV software can detect is quite surprising, especially when it comes to IRC-related malware. Try running some of the AV/Trojan Scanners listed
here.
If you still can't find anything try turning off all scripts (type
/!remote off). If that stops the ident from changing then you'll have to either unload and delete all of your script files or if you've downloaded scripts/addons from several places then you could try and narrow it down by turning remotes on again (
/!remote on) and then unload each script file individually and see if you can find specifically which script is causing the trouble.
