Out of the choices you have there, and going by you pc specs, i would have to say defiently 98SE
Winme = crap (always has been, always will be, ask any I.T "professional"

)
Winxp = a decent os/ but a big no in your case. I wouldnt even install Winxp on a pc with anything less then 256mb ram, and even then, i would be upgrading to 512mb to get some decent speed out of it.
Win2k = also a good o/s (it has to be my fav o/s), but once again i wouldnt install it on your pc unless you upgrade the ram to at least 256mb.
Winnt = Dont even go there, this was never meant to be a "mainstream user" o/s. Since you dont have the install cd, you wont even need to worry about it
Installing 98SE should give you decent speed, whilst still allowing you to use most current pieces of software. If you want to install a newer o/s, i say its time for a hardware upgrade
N.B - Whilst it would be possible to install 2k/xp onto your pc, but its going to run sooooooooo slow that it wouldnt be funny