It sounds like you either toggled the bootable flag off your windows partition, or overwrote your master boot record. My guess is that in Partition Magic, you checked off the box 'make this partition bootable right away' or whatever it was.
Assuming you didn't completely blow away your NTFS/FAT32 partition that XP was chilling on, you should be able to boot off a linux livecd or even the Ubuntu installer disc and install grub ( a bootloader, lets you pick what OS to boot into ). After that, just make an entry for your Windows partition, which is probably root(hda,0).
Once you can boot into Windows, just open a command prompt and type "fdisk /mbr" to rewrite the original master boot record that Windows was using.
Although this all depends on what you actually did and how you partitioned the drive.
Actually, do what dragondude said: continue installing Ubuntu (if you need help post here), and at the end it will install grub and automatically put in an entry for your Windows installation. After that, should you choose to want to remove Ubuntu, just follow the "fdisk /mbr" step up above, then use Partition Magic to add the ext3/reiserfs/whatever partitions from Ubuntu back to Windows.
Posted from an Ubuntu 5.10 installation =)