Look at the registry : [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AeDebug]
You will find the following values if visual studio .NET is the default debugger :

"Auto"="1"
"Debugger"="\"C:\\WINNT\\system32\\vsjitdebugger.exe\" -p %ld -e %ld"
"UserDebuggerHotKey"=dword:00000000

If the value of Auto is set to 1, then the system will automatically spawn the debugger that is specified in the Debugger value.
If the value of Auto is set to 0, then the system will generate a pop-up window.

For example, If you want to use visual Studio 6.0, you can set "Debugger" to :

"C:\Program Files\Microsoft Visual Studio\Common\MSDev98\Bin\msdev.exe" -p  %ld -e %ld

Moreover, you can enable/disable just-in-time debugger from Visual Studio .NET. Go to menu ‘Tools / Options / Debugging / Just-In-Time’ and check/uncheck the following values :

[x] Managed
[x] Native
[x] Script