My avg scanner says I have a virus c:TDD.EXE trojan horse dropper. Agent.cro
Also partition table MBR reads error and boot sector of disk reads error.
What does this mean and how can I fix it?

The bad news is that your computer is infected with at least one virus. The good news is that you can fix it. All of your fixing, though, requires that your computer completely boots so that you can get on to it and do things.

Does it stop booting when you turn it on, or does it finish booting but just gives you all these weird errors?

The best thing is to backup files and format. Although I have heard you can fix virus's in my experience once you get a virus/spyware it's there for life at least until you format.

Get an External HDD firewire and back it up. Harddrives are cheap now, no sense in not doing it.

--FrosT

Backing up to an external hard drive may not solve the virus problem. Some viruses will see that you've attached the external hard drive, and copy themselves to the external hard drive as well.

The solution is to be able to boot the computer into safe mode and run a competent series of anti-virus, anti-malware, and anti-rootkit scanners, but the original poster never replied to my question:

Does it stop booting when you turn it on, or does it finish booting but just gives you all these weird errors?