Trojan Horse/computer security question

> Was wondering if anyone can advise me on a possible
> problem that's cropped up on my home PC.
>
> I use Microsoft Windows XP Home Edition. I ran a free
> security check from Systemac, and it identified two
> Trojan Horses in its system. I was able to delete one of
> them with no problems, but the other, Backdoor.Coreflood
> (in a C:\WINDOWS\system folder the contents of which are
> a complete mystery to me) seems impossible to deal with.
> The problem is that whenever I try to delete the file in
> question, a message comes up reading: "Cannot delete: It
> is being used by another person or program. Close any
> programs that might be using the file and try again."
>
> I'm wondering what kind of a threat this Trojan poses,
> and what I can do about it. According to Systemac, it's
> been on my computer since August this year, but when I
> scan the supposedly infected file with Norton AntiVirus
> it gets a clean bill of health. I've not noticed anything
> amiss while using the computer, and I haven't received
> any mysterious bills for things, but the idea of a hacker
> gaining remote access to one's computer is never a
> pleasant one.
>
> Does anyone know of any way in which I can eliminate
> Backdoor.Coreflood, ideally without having to splash out
> any money?