I also forgot to mention another possible reason for not being able to send email from Yahoo. It may not have been the adware / spyware that MS AntiSpyware detected. It may have been my anti-virus program.

I went to the website of my anti-virus/firewall/anti-spyware software manufacturer and had them perform a scan on my PC for updates. I had some files that may have been corrupted, which were fixed. It is possible that the anti-virus software was causing the problem because it got infected or corrupted.

To check if this is your problem, try deactivating your anti-virus / anti-spyware program and then go into Yahoo and try to send an email message. If you are able to send an email, then try uninstalling the anti-virus program and re-installing it. Make sure to download any updates from the manufacturer. See if you can send email from Yahoo again.