That account is part of the .NET Framework. It's harmless, but if it bothers you, you can disable or delete it.

The spam could be from a lot of things, including the fact that lately spammers have stepped up their efforts in a big way. Though using Internet Explorer and/or Outlook/Outlook Express will not help matters at all.

You may wish to run a malware scan or two, using several different programs. Once you've cleared out all the malware, consider dumping Internet Explorer and Outlook/Outlook Express. Replace IE with either Firefox or Opera, whichever you like best. Email can be done by Thunderbird, or a myriad of other email programs, pretty much any one of which will be more secure.

Depending on how far things have progressed, it may just be faster and easier to format the computer and start fresh. If you do that, be sure you have a firewall running (either XP's or a third party one such as ZoneAlarm, or a DSL router with a firewall) before you ever connect to the Internet. Without all the security updates for Windows and IE installed, you can be compromised in minutes, and have to start all over again.

If you format, the first priority upon getting online should be to hit the Windows Update site, and install all security updates. The next order of business should be installing Firefox or Opera and never using IE again, unless it's to go to the Windows Update site to download new security patches. Never use Outlook or Outlook Express, just install Thunderbird or one of the others from the start. And if you install MS Office, make sure you download and install the security updates for it as well. You can either use Office Update or Microsoft Update, both of which can be found from the Windows Update site.

If you do that, and avoid file sharing programs and opening strange email attachments, you should be able to lead a rather problem free life in regards to your computer.