easy thing to try , control panel>add remove programs>remove yahoo messenger. go to yahoo site, http://www.yahoo.com/
are you using version 7.0 or 7.5 (access this by selecting help>about) ?
download the newest version (not ever a beta) and re-install. se if this helps first.
generally, from what i assume your surfing habits would be like, sites from which i (again assume) you browse contain a ot of spyware and adware and have been known to corrupt, files,programs,reg keys. please try this first. and i suggest running an "adware, spybot" scan for malicious softare detection. you can download these from here----- www.download.com