That isn't a file that comes with Windows. So it came with some other program. To get rid of the message that that program can't find it, your 2 options are:
1. Reinstall that program, or
2. Don't run that program.

http://www.exe-dll.com/dll/rtl120.bpl.htm list some programs that might be involved. Most of them seem to be programs you don't need, but it's not clear from your post which one it is in your case.

Kees