The day before yesterday my Windows XP installation got into a state in which it was unable to start up. It would get half way through startup and then reboot.
Now the same thing had happened a few weeks ago and then I had somehow managed to fix it by starting Windows in safe mode, restoring the system configuration of a few days previously and reinstalling a USB driver and Mcafee Internet Security Suite. At that time I could not tell which of these changes had fixed the problem, I was just glad it was fixed.
When it occurred the second time, I did a system restore to a few days before but that didn't fix it. Then I uninstalled McAffee and it went away! After several hours of fiddling I discovered that it was McAfee Personal Firewall Plus that was causing the problem. If that was installed the startups failed, if it was uninstalled they succeeded. I started poking around in Windows XP error reporting and eventually found an option that switched on or off the automatic reboot on a system crash. Disabling this showed that the error that was occuring during startup was a STOP 0x0000000A IRQL_NOT_LESS_OR_EQUAL, whatever that is. I went to McAfee technical support, downloaded their Virtual Technician and ran it. It reported several problems with my McAfee installation but could only fix a few of them. However, I suspect that most of these problems were because Windows was running in safe mode.
By now it was past midnight and I had had enough of McAfee. I have previously complained about how awkward it was to install on a multiuser PC. Over the past few months I have been using a PC at work that has Norton Internet Security installed and it seemed much less awkward to use. My subscription to McAfee only had another month or so to run. Yesterday morning I walked down to Staples and bought a copy of Norton. It installed without any problems and is now running smoothly and unobtrusively.