hmmm
1st step>>
just insert the windows XP cd and boot from it..while installing XP it will ask you to partition one drive and format it..you have to enter the total space to be allocated for the windows partition in terms of MB's
so if you want 20GB,it should be like 20480 MB
partition it according to your needs..and depending upon the total HDD space you have ...i would suggest anywhere between 20-30 gb if u have a smaller HDD ..or around 40GB..if u have a bigger HDD like 250-320 GB ...assuming that you will install all programs in windows partition which include a few games too..
2nd step>>>
once uve created 1 single partition u will be asked to format it..format it using NTFS file system.after formatting is done windows will copy some files to the hard disk and then the installation of windows XP will start..sit back and relax !
the rest of your hard disk space will remain as RAW...u can create more partitions according to your needs once windows XP is installed from the disk management tool in WIN XP.
hope this helps..any more doubts feel free to ask