Update on progress from last night.
I focused on running the HW tests last night rather than any further OS stuff.
Hard Drive checks came out clear. I ran the Hitachi Drive Fitness Test on both drives and no errors shown. I had already run chkdsk /f and chkdsk /r against both my Vista and Win 7 partitions which had shown no errors/bad sectors.
Memory Tests however did show something.
I started by booting into the Utility Partition and running the Dell memory check utility from there. That showed a Memory Compare error so clearly a problem somwhere.
I then ran the basic suite of Memtest86+ and that threw up over 200 errors on the 1st pass of tests. Looking at the memory addresses listed, it is primarily with Dimm 1 - my first 1gb stick - with a couple on Dimm 2 so this evening's job is remove the first stick and reboot to see what happens. May then remove Dimm 2 as well!
I'll keep everyone posted.