>We have an application in the field that is installed on well over 100 >different client machines. All of the required files to support chm help >are part of our installation package. Some of our XP clients cannot get >help (F1) to work within our applications. The can click on the chm file >and it will launch. Also, we have seen some XP users lose it after it had >been working properly for months and we have had to re-register the help >support files.
>Any ideas what is going on?

The reason may be the sequrity update for both HTML Help and Internet Explorer in XP. HTML Help topics may be wrongly or not displayed if IE is updated and HTML Help not. Ensure that you are using last HTML Help libraries in your installation and also ensure that installer will not overwrite newer versions.

I use following versions of HTML Help files (probably they are last):
hh.exe 5.2.3644.0
hhtctrl.ocx 5.2.3735.0
hhsetup.dll 5.2.3644.0
itircl.dll 5.2.3644.0
itss.dll 5.2.3644.0

Also ensure that you are properly installing and registering foxhelp7.exe and foxhelpps7.dll.