I've stumble upon an instant messaging client that is has a very low fingerprint on the operating systems resources. It's open source and it even runs directly from an USB memory without any installation. It's called Miranda and it supports protocols like:
AIM (AOL Instant Messenger)
IAX (Inter-Asterisk Exchange)
IRC (Internet Relay Chat)
I find it like a very lite version of Pidgin (pidgin.im).
You can download Miranda IM and some add-ons from here.
It's not fully mature yet so you may find it far to simple compared to Yahoo's Messenger application. But it's small, fast and has some nice features like transparency, multiple protocols and accounts, auto-reply, nice idle and status options and so on. The downside is the fact that compared to Pidgin, it only runs on Windows 95 / 98 / ME / NT / 2000 / 2003 / XP / Vista / 7. No native Linux releases yet.