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It would work if you didn't use the proc and endp macros which modify the stack. See, if you call something then it's return address gets pushed onto the stack. Here, you pop that return address into eax inside your convert proc. Just change your Convert proc as shown below and all should be fine.

Convert proc myflag:DWORD

	invoke StrToFloat, addr buffer, addr inputvalue
	fld inputvalue
	.if myflag==0
		fmul SEKvalue
	.else
		fdiv SEKvalue
	.endif
	fstp inputvalue
	invoke FloatToStr2, inputvalue, addr buffer
	ret

Convert endp

A good exercise would be to debug starting from push eax | call Convert and see how exactly everything works ;)

Anything that gets PUSHed outside of a CALL should to be POPed outside of the CALL to keep things clean. This is the standard for CDECL (LibC) and FASTCALL (64-bit replacement for Win32 STDCALL, which in itself is actually more like CDECL.) However, if you don't need the value that you PUSHed, simply ADD to ESP.

			invoke IsDlgButtonChecked,hWin,IDC_RBNEUROSEK			;get checked state of Euro->SEK radio button
			.if eax==BST_CHECKED
				mov edx,0
				push edx
				call Convert
				add esp,4    ;***** Compensate for 4-byte PUSH of EDX prior to CALL *****
			.endif
			invoke IsDlgButtonChecked,hWin,IDC_RBNSEKEURO			;get checked state of SEK->Euro radio button
			.if eax==BST_CHECKED
				mov edx,1
				push edx
				call Convert
				add esp,4    ;***** Compensate for 4-byte PUSH of EDX prior to CALL *****
			.endif

Also, to access values on the stack, use ESP/EBP...

Convert proc

	invoke StrToFloat, addr buffer, addr inputvalue
	fld inputvalue
	mov eax,DWORD ;***** EIP takes up closest 4 bytes ATM, go beyond it *****
	.if eax==0
		fmul SEKvalue
	.endif
	.if eax==1
		fdiv SEKvalue
	.endif
	fstp inputvalue
	invoke FloatToStr2, inputvalue, addr buffer
	ret

Convert endp

HtH.

Just keep in mind that with the stack, First In Last Out (FILO) ;)