Thanks for the responses.
ante - I have 2 hard drives, C & D. XP is on C and W98 is on D. So I assume that this is not the issue. There was no problem initially after having the computer up-graded in January. And I previously had W98 & W2000 operating without a problem.
plot - I can't remember exactly when the XP problem started or exactly when I installed SwitcherCADIII. All I know is that the time frame is similar. In other words, I don't really know if the XP problem started before or after I installed Switcher.
Yesterday, I did a Disc Clean-up and then looked at XP Help & support. It said that there is a "Support Tools" option on the XP disc and to do this and that to load. So I inserted the disc and could not find "Support Tools" but found something similar for loading extra functions. I tried to select what I wanted but things changed and I don't know what it loaded. However, it asked me to restart. So I did and hey presto up came XP. I had left the XP disc in the CD reader and it took some time to come up so it must have been installing something???
The XP disc was still in the CD reader when I switched it on this morning. When the XP screen appeared with the running blue (I thought they used to be green) squares - the squares froze and the CD reader light came on. After a few seconds the CD LED went off and the squares started moving. This happened several times. So I assume it was taking something off the disc. It took much longer than usual to bring up the blue "welcome" screen but it did not fail.
Then while using PowerPoint, there was a brief power failure and the computer shut down. (I had removed the XP disc earlier) But when I switched it on, it cam up normally. So I will wait a few days to observe the boot ups before opening Switcher again.
Len