Retail is aimed at the end user consumer and has full microsoft support. OEM is aimed at computer builders and most of the support ( if any is needed) is expected to be dealt wih by the builder.
retail: it comes in a nice box, with fancy user manual and has tech support
oem: (original equipment manufacturer) usually for dell, hp, etc. they have their own manual, and tech support. this is usually tied to a proprietory manufacturer hardware (such as cpu serial number) chipset or bios. this may not work with home-made pc's. (hardware lock).
oem for public use: (for people who home-build their pc's) same as above without hardware lock.
corp edition: provided to big companies without activation. can be installed on as many pc's as the company wishes (costs a lot).
volume licensing edition: same as above (some difference). comes with cd-key and c.o.a. (certificate of authenticity).
your best bet would be to buy an oem version with cd key and c.o.a. from places like here.
it's win-xp-pro-sp2 and it costs about $70-$80 and it works fine. no activation is required.