Spyware?

Hi, how are you?
I'm having a strange problem with some of the PC's here at my office.
I've seen a file named '~' on my desktop with a lot of garbage and e-mail's.
I've checked with Ad-aware, Spybot and HijackThis and found nothing.

Also I've found on one of the computers two files called diaA and a number with exe extension.

I've also did a scan with AVG up to date and it found nothing.

Any idea about what could have done that would be appreciated.

BTW all PC have win98se, IE6 SP1 with the lastest patchs

Thanks a lot.