I just recently bought a mac after my PC died, and I want to back up my computer on the external HD (it’s a Maxtor 400GB OneTouch) and Time Machine says I need to reformat it, but I’d rather not lose my old files.

I read about partitioning and I’m not completely sure if it’s a good alternative to formatting. If anyone could give some input, I’d really appreciate it :]

If you partition a hard drive, the new partitions which are created will "usually" be formatted. There are some software that support partitioning without formatting (eg: norton partition magic). But i don’t know if there are any for Mac.