I ran 3d Mark 06 with the graphics test, and the graphics card is performing poorly too. This has led me to believe there is a problem with the PSU. Is there any way to test the PSU to see if it is functioning properly?
(If it helps, CPU-Z reports a V-core of 1.3120 even under 100% load during superpi. The normal is 1.3500)
Thanks for your replies, Technology Coordinator, and especially for being skeptical about the PSU.
I fixed the problem by system restoring to earlier today, when it was working. Evidentially something I had installed between then and now caused the error. Restoring it removed this error, and now the system works properly.