I imagine the reason the back button is disabled has something to do with the virus you have HTA is HTML executable file. at a guess I'd suggest it was doing something clever whilst sending out information to other people. it's constantly in use which is probably why you can't delete it.
Mcaffee will have no joy deleting thie either.
As far as I can think, you have two options.
1, tell spybot to run when the computer starts, hopefully it will detect and clean this file before it loads into the memory and is then protected by the OS.
second option is download a copy of Hijackthis.exe
. run this and tell it to remove the process. this will almost certainly work.
The virus will almost certainly becalled trojan.something or other, the only other reason you wouldn't be able to delete a virus is if it was in a protected folder such as the system restore folder.