Many thanks for all the comments. The computer is a home build and the board is about 5 years old and after using Vista has been running Windows 7 Home Premium 64-bit since just after it was launched with no problems until recently. I did check at the time that the board was compatible with Windows 7 and I think I flashed the BIOS at the change to 7 - can't remember for sure.
PSU is an Antec TruePower, 850w if I recall correctly. The drives are using the nVidia ports - not using RAID. The one with the OS on it is is the same age as the board - 5 years but all safely backed up. Sometimes when the computer freezes the hard disk light stays permanently on, sometimes it is off.
I've turned off my weather station software as I'm deeply suspicious of it being the problem and will see if I get any freezing for the coming week. It's been running on an old XP laptop for the last 4 years - quite often with different problems to the current one - and was only transferred to this computer three months ago. If the freezes still occur during the coming week then I'll set about reseating everything and running memory and drive diagnostics.
Last edited by CharlieOneSix; 16th July 2012 at 10:48.