Windows XP is an operating system and has nothing to do with emails.
You can access your emails via Outlook Express which is an email client built into Windows XP (that is to say it's not an email itself but can be configured to send and receive emails when given the correct details).
Can you give more information? What email is it? Are you using Outlook Express?
Sorry... I just read the title of you question is "Yahoo" ... so I guess you're using a Yahoo web-based email?
What error messages are you getting? Are you using the "normal" version or the trial beta version? I recently reverted back to the original as I was having some problems with the beta - hopefully they'll be sorted soon!