problem with partition

hi, i wanted to partition my drive so i can have xp. the first problem i encountered was that i didn;t have that much free space. after cleareing some free space i had 100 gb/140 gb. when i went to shrink it said that i could only shrink 6 gb. why is this so (wanted xp to be able to use at least 70 gb). When i shrink a partition is that the only space i can use for my other operating system? I went ahead and tryed it anyway but when i pressed shrink it said in an error box, Access denied!

Please can someone tell me what i should do and how i can dual boot vista with xp already installed?

(I own xp CD, but not vista CD)

Please Help

There are some important limitations to the Vista partitioning tools that you should keep in mind: you can only shrink and expand NTFS or RAW (unformatted) partitions; when you shrink, there are some unmovable files such as page files or shadow copy storage areas that inhibit the amount of shrinkage allowed; you cannot extend a partition into unallocated space to the left (lower disk addresses) of the partition to be added to; you cannot move around unallocated space; if you have too many bad clusters on the disk, then you cannot shrink a partition on it. Many of these limitations are removed by commercial products such as those from Symantec and Acronis, or by Boot-It NG, which I will describe next...

Boot-It NG is a shareware partition management utility that can be used to manage the shrinking and extending of partitions better than Vista's built-in tool. It has an old DOS style interface, but it works well. You can get it here:
Boot Manager, Partition Manager, and Drive Image Utility - BootIt Next Generation
I downloaded the free trial version and burned it to a CD with the included program MakeDisk.exe in order to be able to move some unallocated space immediately behind one of my four partitions that I wanted to extend. I just booted from the CD, clicked on the menu choice for "Partition Work", and then used the "partition slide" concept to move the unallocated space. The TerabyteUnlimited site has a good deal of documentation, tutorials, answers to FAQs, videos, etc. for how to use the program:
TeraByte Unlimited :: Support :: BootIt Next Generation

As far as dual booting: This page describes doing it with Vista installed first and then XP afterwards:
How to dual boot Vista and XP (with Vista installed first) -- the step-by-step guide with screenshots
How to dual boot Vista and XP (with Vista installed first) -- the step-by-step guide
One problem with dual booting XP and Vista is that there is a bug in Microsoft's implementation of XP such that any time you boot to XP it wipes out Vista's System Restore points. There is a registry workaround that can prevent this:
No restore points are available when you use Windows Vista or Windows Server 2008 in a dual-boot configuration together with an earlier Windows operating system
No restore points are available when you use Windows Vista or Windows Server 2008 in a dual-boot configuration together with an earlier Windows operating system

thank you so much for the advice :-)