ReadRegStr maximum lenght?

[MeRuud]

Dear all,

I am trying to add a check to see if the software is installed or not. If it is installed the installation should abort, if it is not installed the installation should continue as planned.

I currently have this:

; Check to see if already installed
Section -Run
ReadRegStr $R0 HKLM SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\{BF61699A-6D79-4A97-B879-7A890E5A9A48} InstallLocation
IfFileExists $R0 +1 NotInstalled
MessageBox MB_OK "Driver is already installed, installer will continue."
Quit

NotInstalled:
MessageBox MB_OK "Driver is not already installed, installer will continue."
Quit
SectionEnd

This does not work, register (and its key value) is not recognized.. It simply says that it is not found (but it is definitly there)

However if I try:

ReadRegStr $R0 HKLM SOFTWARE\Microsoft\Windows\CurrentVersion\ ProgramFilesDir

or

ReadRegStr $R0 HKLM SOFTWARE\Microsoft\Windows\CurrentVersion\Themes InstallTheme

It does work properly.

I also tried:

ReadRegStr $R0 HKLM SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\AVG InstallLocation

Which doesn't work either.

Is there a maximum lenght of the ReadRegStr? Can I only enter so many subfolders?

Thanks in advanced,
Ruud