If you can get to Yahoo but not to your emails, it suggests that you've lost a "login cookie" that you may have used to send a password automatically. You may just need to log in manually.
That should NOT affect getting to everything at mudcat. You may need to reset the "permissions" in your browser to accept (and keep) cookies from mudcat to get full use, but just reading doesn't require any permissions with common browser setups.
If your computer is running a lot slower, and you've done Disk Cleanup, Defrag, etc, it suggests that something is running on your machine that shouldn't be there. If your AV has expired that's also a good indication that you may be a victim.
You can look in task manager for clues, but unless you're familiar with what's supposed to be running it may be hard to spot anything wrong.
Most AV publishers offer a free on-line check. You don't usually have to be a user of their product to run one, although some require you to have a current version before it will fix what gets found. Your own AV program's site would be first choice, but I've found Norton Security Check to be among the good ones if you can't get to another that you prefer.