Network grinds to a halt

We have peer to peer network (4 PC's, 2 x Vista, 2 x XP plus 1 HP printer
sitting as a network device)
At this time only the two Vista are used and are used intensively but both
PC's have 2GB memory and are new. In booted idle state all PC's can easily
see and browse the default shared public drives on the network and all these
drives are mapped to each other. At least once a day the network grinds to a
halt and files saved to a mapped network shared drive take forever to save,
if at all. This target vista PC is used rather much with Adobe Photoshop with
largish images up all over the place but these images are not dragged over
the network.. Today that target network drive on that Vista PC could not
even be opened itself (never mind over the network). PC has to re-booted. I
had the HP printer USB’d to the “Adobe� Vista PC and that took long to print
over the network (hour glass would take 20-30 seconds to release); luckily it
was able to connect directly to the network and printing became almost
instantaneous. Thisperhaps points to an issue with the Adobe PC. Is this a
memory issue perhaps on that one “Adobe� PC; most network activity is opening
and saving SMALL word and excel files to this Adobe PC; I am not if there is
problem the other way around. I thought 2gb would be enough. Any tools I can
use to diagnose to see if it’s the PC or something on the network. Thanks