Mailwasher Pro is indeed the way to go. It enables you to check your mail whilst it is still at the ISP server. You can delete and bounce anything you don't care for, and download the rest. It may take a while to overcome the problem you now have, but in time you should be pretty well rid of the stuff.
My current ISP (Internode) has a excellent spam filter anyway. It needs to be checked from time to time, because just occasionally a useful email can be trapped there as a false positive.
It sounds like you've done most of the things needed to check your machine for nasties.
Malwarebytes is an excellent checking program which you may care to also download and run, just in case AVG has somehow overlooked something.
If your machine passes that test also, I would be inclined toward a fair degree of confidence that all is well.
FWIW
FOR