[kichik]

code:
StrCmp $1 "" 0 loop

This would be much easier with the LogicLib:

code:
!include LogicLib.nsh

#...

StrCpy $2 0

${Do}

	ClearErrors

	EnumRegKey $1 HKLM SOFTWARE\Microsoft\.NETFramework $2

	${If} ${Errors}

		${Break}

	${EndIf}



	${If} $1 == "v2.0.50727"

		MessageBox MB_OK found

		${Break}

	${EndIf}



	IntOp $2 $2 + 1

${LoopWhile} $1 != ""

Or even better:

http://nsis.sourceforge.net/Get_.NET_Version