Though hex numbers can be converted to decimals using SET /A MY_DECIMAL=0xFF, the other way around is not natively supported in Windows batch. However, the following subroutine does this job for numbers 0 <= x <= 255 (8bit).

:: Converts a decimal number to hex. Currently only numbers 0 <= x <= 255 are 
:: supported (0x00 <= x <= 0xFF). The result will NOT be prefixed with 0x!
:: 
:: @PARAM Integer decimal number 0 <= x <= 255
:: @RETURN String hex representation of number
:: @SINCE 1.0
:DEC2HEX
SETLOCAL ENABLEEXTENSIONS ENABLEDELAYEDEXPANSION
SET HEX=0123456789ABCDEF
SET /A T1=%1 / 16
SET /A T2=%1 %% 16
SET RESULT=!HEX:~%T1%,1!!HEX:~%T2%,1!
ENDLOCAL & SET RESULT=%RESULT%
GOTO :EOF

Note: This subroutine is part of the number.cmd library contained in the batchlib package since version 1.0.