This question has been arising more commonly since around October 2001. 9 times out of 10, it's due to the virus scanner in place.
With the current version of Norton AntiVirus, there is a new feature that blocks script access. The scanner thinks that your IUSR_MachineName is doing something nefarious. You may turn the "script blocking" feature off. View this link for more details:
The solution has various outcomes. Some users turn off the blocking, and voila, everything is fine. Others have gone as far as completely uninstalling Norton in order for the FSO to work.
To my knowledge, this hasn't been as big of a problem for Mcafee users. If so, please check out McAfee's web site here.
If you're sure your FSO code is correct, and you're still experiencing problems, and a virus scanner is in place, your best bet is to contact the manufacturer's technical support.
UPDATE - August 2003 Some alert 4Guys readers have written in to mention that they have experienced this problem with McAfee VirusScan, in a feature called ScriptStopper. This was fixed by disabling ScriptStopper from the Options/Advanced menu.