One thing I have noticed is that if I for instance copy an e-mail address from the header field displayed in Yahoo! Mail and try to paste it into a new mail the IE tab or windows containing Yahoo! Mail becomes unresponsive. Let's say I have an e-mail from someone and want to copy and paste the e-mail address visible in the header field above the actual e-mail into a new e-mail the window freezes up and I have to reload Yahoo! Mail. What I have had to to is to copy the information from Yahoo! Mail into Notepad, and then from Notepad copy and paste it into the new mail where I need it to go.
I am using IE7 on Windows Vista Ultimate, and I only have this problem with Yahoo! Mail. There's no such problems in Yahoo! Mail Classic.