I have the new version of yahoo on my pc - Windows 7 operating system and have no problems with it.
Some suggestions - please keep in mind that I am not an expert - some would say I have enough knowledge to make me dangerous!
Use a "Restore Point" on your computer to reset the software that runs the computer to an earlier version. I don't think that will remove the new version of yahoo, but it will change the system software back to what it was before the yahoo update.
If your anti-virus software isn't up to date, you may also have picked up a virus - not necessarily from downloading yahoo.
If you have more than one anti-virus software running on your pc, that can slow the PC so much that it seems like it is stopped. They fight each other for memory to do their anti-viral thing.
good luck!