Internet failure

Large network with everything running XP from NT servers (going to upgrade
but need this solving first!!). The logon script assigns network drive to
users depending upon their needs and requirements but the common thing is
that there is a call on an *.reg file to alter the proxy settings in IE to
enable internet access. The files looks like this

REGEDIT4

[HKEY_current_user\Software\Microsoft\Internet Explorer\Main]
"Start Page"="www.intra.leedslearning.net"
"NoUpdateCheck"=dword:00000001

[HKEY_current_user\Software\Microsoft\Ftp]
"Use Web Based FTP"="yes"

[HKEY_current_user\Software\Microsoft\Windows\Curre ntVersion\Explorer\Tips]
"DisplayInitialTipWindow"=dword:00000000
"ShowIE4"=dword:00000000
"DisableStartHtm"=dword:00000001

[HKEY_local machine\Software\Microsoft\Windows\CurrentVersion\ Internet
Settings]
"ProxyEnable"=dword:00000001
"ProxyServer"="4105proxy.intra.leedslearning.net:8 0"
"ProxyOverride"="owa.intra.leedslearning.net;stack er;lln-wsgt-ig*.leedslearning.net:8002"
"ProxyHttp1.1"=dword:00000001

Sometimes the script takes but other time it doesn't. If I as administrator
bypass local security and run the logon script again, there isn't a problem
and the user can access the internet.
Any ideas how I can permanently fix this so that I do not need to keep this
*.reg part of the script.