The temp files are just for images and sites visited. Nothing in the temp files will contain a email viewed. The temp files are just for when you go to the same sites teh images dont take as long to load.
If you have downloaded the email it should be in a place you specified. If you use a email application then it will be withint hte profile of that application.
If you dont believe me check for youself. Open My Computer. Tools>Folder Options>Show all files. From there go into you XP Drive (should be C
and then the Documents and Settings. There should be a folder called Local Settings. In the Application Data is where all your profiles for specific applications are stored. The Temp are the hidden Temp files.
You can search that as much as you want but unless you use a specific tool to download the email to Outlook or Thunderbird or Outlook Express your outta luck. Sorry. If Yahoo cant recover the email from their Server you have to get it resent.