Try web-based yahoo client. (Use your yahoo user id and password to login)

1. www.meebo.com
2. www.e-buddy.com
3. www.koolim.com
4. iloveim.com
5. http://express.instan-t.com/myim/start.htm

Most of them support other client too such as MSN, ICQ, IRC, Gtalk etc.
Dont forget to share your experience here.

Thanks, vangardx