I have a problem starting the machine this morning. Closed down normally last night. Now on switch-on, I get the following report: "Radeon X600 SE P/N 113-A26044-107 BIOS". System hangs and will not go into Windows. Have confirmed all cable connections are okay.
Dell XPS/Dimension 5150C Windows XP SP3 (fully updated) Zone Alarm Internet Suite (fully updated) Webroot Spysweeper (fully updated)
Iarno's advice did the trick - removed and replaced the memory sticks. All is now well.
howard64 - tried to restore via the Acronis original disc but system would not start, presumably because of the memory failure check. But I must check BIOS to ensure that 'booting from CD' is the first at startup.