I really doubt that 750mb of mails would slow the computer unless you have a very tiny hard drive. That is only .73Gb. What WOULD slow your computer is:
Files Infected: 1347 for starters. The big one was adware.zango
Adware.ZangoSearch is an adware program that monitors the contents of Internet browser windows. It opens the Web pages of partner sites when certain keywords are detected in Internet search or shopping browser windows. This program also installs files related to Adware.180Solutions. You also had a Backdoor.Bot on the computer.
Backdoor.Bot is a backdoor trojan that can give an attacker to gain remote access on compromised computer without users knowledge

You have NO anti-virus program on the computer, you have NO FIREWALL on the computer. Some of these infected files were found in some mails. Some were in smilies, etc.
Your Java is WAY out of date. You have an old version of Internet Explorer and this with the out of date java and NO anti-virus or anti-malware programs (except MBA-M which is obviously new) on the computer has left the door wide open and nasties have certainly come in you are lucky this is all that has been found...so far.

ESET scan found a combofix quarantine file...when did you run combofix?
Looks to me like the only thing you have updated may be the OS, and of course that was only partially since you have the old Internet Explorer on there. IE7 has been out for ages, heck they have all ready released IE 8 (DON'T INSTALL THAT)
With all this Zango malware on there it probably damaged the Firefox. I don't see anywhere that Flash was removed with the scans so it was probably damaged also.
I would like for you to run at least TWO more of the online scans listed in PP's Sticky
Please have them clean if they offer it. House Call will clean I am fairly certain. If they don't offer cleaning then save the logs.
After you do those, be sure to reboot after each by the way, then Update MBA-M and run another Full Scan with it. Have it remove all it finds. Reboot.
Run a new HJT scan after all of the above and copy/paste all logs back here. Please don't attach the logs. Copy/Past them
Judy