new software installed: registry broken

When I installed new software recently, after rebooting the system I got the error message: Error: real mode system memory allocation failed.
If I then reboot to a DOS prompt and restore a previous registry, the machine starts OK but of course cannot find the new software. This happened with internet explorer 6.0, and also with wordperfect 2002. When I check my system for errors I find none. My disks are also defragged regularly, and the system works fine otherwise. I have up-to-date virus protection and the software reports no viruses.