The +R/RW and -R/RW are simply two different formats, much as VHS and Betamax were in the old days of video. +/-R means write once only , and +/-RW means ReWritable. Which format you use, can depend on which make of drive you have, but many drives now support both formats. You can find out much more information on the different formats
HERE. You might be able to find out the make of your drive by looking in Device Manager, under DVD/CD ROM Drives. To find Device Manager in XP, right click your My Computer decktop icon, then select Manage, and you'll find Device Manager in the next window which pops up.