I know I can have two partitions on my hard drive and have win xp pro on one and Ubuntu on the other.

And just set it up as a Dual Boot system.

But I have win xp pro on my System now and it was setup with one Partition.

Can I somehow make a secound Partition and install Ubuntu and have a Dual Boot system or is it to late I have to just refomat and start from there??

Just put in your install disc, and instead of deleting or over writing, just install on a new Partition

You need to create a partition then install the second os to it.

How to dual boot Windows XP and Linux (XP installed first) -- the step-by-step guide with screenshots