If I shut down the PC and re-boot, all is well again - until the next time. It seems quite unpredictable and random.
I'm using XP Home (SP2) and Word, Excel etc. from Office97. I know about the caveats installing Office97 in SP2 and have heeded them - I don't think that's the issue as this is a recent problem (the programs have been installed for nearly two years).
Any help would be much appreciated.