Think you kinda answered your own question, use the KeyExist function ...

Code:

Break On

$MS05_051 = KEYEXIST("HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\HotFix\KB902400")

IF $MS05_51 = 0
 $MS05_051_S = "Good"
ELSE
 $MS05_051_S = "Bad"
ENDIF

?"Value=" $MS05_051_S

Exit 0