you could totally backup your history each month or week depending how large it gets.
Then burn it to cd.
Then clear it out.
when you burn it to CD don't finalize the cd that way you can keep adding to it each week.
have 100 directories with each persons name for the directory then put all convos from that person in each directory.
I think the history is in this directory
C:\Documents and Settings\YOUR_USER_NAME\Application Data\ICQ
If you want a small batch file or something that will back it all up at the click of a mouse let me know and I'll see what I can do
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