Machine-dependent Macro performance

I am having trouble with the following section of code:

Open/Z/R/P=dataPath Filevar as FileName
FStatus (FileVar)
if (V_Flag==1)
FSetPos (FileVar), (V_logEOF-15)
FReadLine/T=(num2char(13)) (FileVar),ReadStr
df=str2num(ReadStr[strsearch(ReadStr,"=",0)+1,strlen(ReadStr)])
else
Abort "FILE NOT FOUND"
endif

I am running Igor Pro 6.0.3.1 on two nominally identical ThinkPad T61
machines running Windows XP Professional Version 2002 Service Pack 2.
Here's the problem: On one machine the code works fine; on the other
machine for some reason V_Flag never gets set to 1 so the code always
aborts. Any ideas?
I doubt this has anything to do with the machines themselves. My guess is that in one case the file is being opened and in another case it is not. You should check the values of V_flag and S_fileName after the call to the Open operation, and see if that gives you more insight. You might also try this without the /Z flag for Open to see if that gives you a run time error that's useful.