[Phil^]

look at the hardware listed for it ( windows-key and pause/break key at once for shortcut ) via device manager
the cpu clock and memory will be listed on the first page, with all other hardware devices installed listed under the device manager itself.
the hard disk space you can verify via the disk management tools ( start, run , "diskmgmt.msc" )
that'll show you the partitions on the disk, capacity etc.
Bear in mind hard disks count in factors of 1000 for mega -> giga and not factors of 1024 as it should be, so a 60 gig drive will appear as something like 55 gb or something because of that and stuff.
the monitor you will just have to judge when you are there, same with printer, speakers etc.
As for viruses, as long as you get the windows install disk and driver disks you can just wipe the thing yourself and be certain.
It sounds so-so, something like that i would price at about £120-150 though if i were to sell it but it depends how many accessories and software is being thrown in too