Thanks for the reply!
I am in the process of trial and error diagnostics.
The system had 1Gb DDRII 533Mhz memory. I bought two more and have rotated them all in and out in different pairings and as singles and I still get the lockups.
I've re-formatted the hard drive again and re-installed Vista (which thankfully is a relatively quick and painless affair compared to XP!

) but only allowed the system to download and install the high priority windows updates. There are currently three optional updates available one of which is a set of drivers for the WLAN. I've deliberately not downloaded these and am getting no lockups so far....
Rez