Sorry to take so long for my response. I have been on a long, upsetting, and thus far unsuccessful journey to figure out what's wrong with my system. I now know the following:
1. My problem of Avast not loading is not unique to Avast. In fact, there are several things on my system that will no longer load. These include but not necessarily limited to: Windows Help and Support (from the start menu), system information (from start-->accessories-->system tools), and Ulead DVD Movie Factory (a program for making VCDs, SVCDs, and DVDs). In each of these, as well as Avast, attempting to load results in absolutely nothing. The system hangs for a sec and then nothing happens. However in task manager, I can see that a new process has been loaded.
2. Using run-->msconfig, I have selectively gone through and prevented almost every potential culprit from loading at startup, in an attempt to pinpoint what might be the problem. This includes all the HP printer stuff, my logitech mouse, windowblinds, my nvidia audio stuff, and many other things but to no avail. In fact, even if I select "Diagnotic Startup" such that only "basic devices and services" are loaded, the problem still occurs.
3. The problem occurs no matter which user I log into in WinXP. There are three user accounts in total, though only my main one gets used very much. But in any of the three, I have the same problem of not being able to load certian programs including avast.
4. The one and only way that the affected programs will run, including avast, is to boot in safe mode.
At this point, I really have basically given up trying to figure out what has happened. I can probably live with it for a little while longer, and once I have time to get all my files and stuff organized and triple backed-up, I will probably format and reinstall windows from scratch. And I hope to install avast right from the start and not have any problems with it, or with anything else. Thanks for all the help and suggestions though. Much appreciated.
