As above I only have one account with XP, however, if the reasoning behind user accounts is anything to go by, having two separate mIRC's installed in two different user accounts should effectively be the same as having two different computers with two different mIRCs - Unless of course the files are in the "shared folder".
You can control user accounts in Windows XP through Start > Control Panel > User Accounts
What did you mean "Which programs to open"?
Might also like to take a look at this thread
, and if you think it's a worthwhile idea, post it in the Feature Suggestions forum and see what others think - Khaled will also see it. Can't remember if it's been directly suggested before.