This evening I removed the first memory Dimm in slot 1 - reran Memtest86+ and that ran completely clean. Rebooted the machine and Vista booted straight up. Not tried 7 yet but guess that will likely be ok too. Now just need to do some memory movement to test a working stick in Dimm 1 and see if tis ths stick or the slot that is faulty.
Thanks all for the help - a faulty memory stick wouldn't have been my first guess given the symptoms presented but one for me to remember for the future.