Most cases that I've seen where Windows locks up and won't respond to Task Manager or to repeated Ctl-Alt-Del to reboot appear to come from memory overwrites. One program writes into the RAM area that another program is using, and when the other program that thinks it owns that space tries to fetch what it put there, it gets garbage from the program that overwrote it.

This should be quite rare with programs that are "approved" for the Windows version you're using, although even if you use only fully qualified programs it can happen. It should be extremely rare with certified programs, especially on WinXP.

The few "approved/qualified/certified" programs that cause lockups are, in my experience, nearly always games; but others cite some add-on "utilities," especially when you have lots of them. WinXP (especially in Pro) generally has built in diagnostic and management tools that are much more complete than in prior versions, and if you can find them and learn how to use them, there's very little real justification for "extra helpers" that may have been necessary with earlier versions.

Lots of programs that run quite normally are not "certified" compatible, since that requires submitting code and testing results to Microsoft and getting them to agreee - and costs a little bit. Programs from reliable vendors who assert that they're "for WinXP" generally can be trusted, so it can't be said that you can or should only use stuff that Microsoft has approved, but you do need to be reasonable about it. A lot of freeware/shareware is reasonably compatible, but some isn't. Excessive reliance on "special programs" to do things that WinXP is fully capable of doing for you is a common cause of "vague and intermittent problems."

Programs carried over from an earlier Windows version may have problems running with WinXP, since there are some differences in how processes are executed in WinXP. WinXP does not permit programs to access either the hard drive or RAM directly without specific program features that were not required by earlier Windows versions.

Copying a program that was installed under an earlier version often will not get the needed Registry entries to permit and properly to keep track of the direct memory accesses that older programs used fairly frequently, when the programs are run in WinXP and in the presence of other programs running at the same time.

WinXP also occasionally wants bits and pieces of the program to be in specific locations outside the main program folder. It usually will "hunt up" a file that's not where it should be, but sometimes - especially with a .dll - it may create a new one, resulting in duplicate and sometimes conflicting files.

It's generally recommended that programs should be installed, usually from the original program disks (or from a downloaded "install file,") in order to "register" the functions the program requires so it will run nicely with other programs.

Some programs are simple enough that copying the program files from one drive to another won't do any damage, but it's difficult to be sure when that's true and when it isn't. Sometimes you can set the properties on a program you brought over from an earlier version to have it "Run As Win98" and performance will be improved - or at least interference with other programs will be minimized; but it's difficult to tell when this is really going to work.

Some older programs that appear to run properly without being limited with a "Run As" property may actually interfere with other programs, and locking them down to the earlier version mode may help prevent the interference - but it's difficult to tell when this will help.

John