If Yahoo Messenger puts enough of a strain on your system to slow it down ten you might want to upgrade to a computer built in this century. I mean, what kind of chip do you have in there, a Dorito? Anything over 1GHz won't even notice the CPU load, and anything under that was made over a decade ago.

Or it is possible that you just don't have enough RAM to even run Windows right. Realistically, WinXP requires 1GB and Vista 2GB if you wantto run anything other than just Windows.
If you have a 512MB system, you can't even hold all of the processes for WinXP in physical RAM and have to rely on Virtual Memory, which *will* slow you down no matter what, even if you are not running ANY applications.

Or maybe you need to defrag your hard drive. Many people don't do that and then wonder WTF when their system gets slower and sslloowweerr and ssslllooowwweeerrr.

While any processes running will use resources, you won't see a significant difference when using Yahoo messenger. Unfortunately it seems your friend is just guessing without actually having performed any analysis into the problem.

Yes, he's right it does slow down your computer. AIM will do the same. I'm not to sure why it does, maybe because it's a lot for the computer to process.