You absolutely can do what you wish, but will need third party software.
Here's the deal: While XP does include a disk management tool, it cannot
change the sizes of existing partitions without data loss. To change the
sizes of your partitions you need a third party tool, like Partition Magic
by PowerQuest (which was recently acquired by Symantec).
Partition Magic is very reliable software, but you do need to read the
documentation very carefully before you start. Ordinarily, PM is about $80
(in the US), but you can find it for much less. There are other tools that
are less expensive, but for the average user, PM is it.