As others have said, Pentium M is not a Pentium 4. You could comfortably run Windows ME, Windows 2000 SP4 or Windows XP SP2 on a Thinkpad R51. I am posting from a Thinkpad T42 running Windows ME with the unofficial service pack actually.
First thing you'd want to do increase the RAM. If using WinME, go with 512MB. For Win2k or XP, max out the RAM, probably 2GB at most. Lenovo still has drivers for their older laptops on their site, but usually their driver support only covers Windows NT derivatives (2K, XP etc). In my signature is a link to Thinkpad T42 drivers for WinME, they might be of use to you.
You could technically use Windows 98SE, but it's hard to work with because it doesn't have USB Mass Storage support out of the box. Without that, it's tough even finishing the Win98SE install because often it won't have any drivers for your Pentium M chipset or CD/DVD drivers. If that doesn't end up happening, you'll still be without an easy way to load drivers into the laptop, unless maybe you have a floppy drive or something and can deliver the USB Mass Storage driver that way.
If you have an actual Pentium 4 laptop (which I do), although yes it's a great personal space heater, it's still a decent enough laptop as long as you don't run heavy tasks on it. My only beef these days with Pentium 4 is it just feels slow compared to the Pentium 3. Do not run Windows XP on a plain Pentium 4, that is pure torture. Bare minimum should be a Pentium D, which is a dual core P4. I recommend Windows 98SE, WinME and Win2K on a Pentium 4 laptop.