Too many unallocated drives with 0mb size. How to delete them? Help tech guys.

Hi, here's another problem that my pc came up recently.
I installed xp and Vista on the same HDD and i forgot to install my antivirus when a virus came and totaly wrecked my pc up... When the virus was installed... i couldnt do anything and my explorer couldnt open even with safe mode. I installed xp on an pre-unallocated 5GB size and booted from there to fix the problem. i deleted that virus and the PC wouldnt start even with the recovery disk. so i had to backup my music and pictures to complete delete the volume and make a new one.
Through many dificulities i managed to install Vista back and delete the xp on the partition to enlarge the size of it and have both OSs on 1 hdd. Now here starts my problem.
Vista wont allow me to make more that 7 or 8 Gb of partition size, so i make 3 partitions of 7GBs and then delete them to create a brand new 21GB partition, (and if i delete that partition and stick it with the main HDD, and ask to shrink partition it allows me 21GB, i want an answer here, why in the begining 7 and now 21?)...
I try to install XP and it loads everything fine,but then, some where in the end of loading setup files it ask me to wait 15 secs to reboot ot press enter to reboot now...
It restarts, and loads back to Vista! Why? It shouldnt normaly do that because i ve made numerous of formats in the past.
I try to set the boot priority to DVD/CD and again i get the same results.
I am very suspicious about these 0MB of unallocated HDD space.
What do you think guys?
Ow and by the way, i am runnin on this pc the Vista perfectly and each time it would format it without any probs.
You gave me a page to take a look of how to have both OS on a pc and it didnt helped that much because i knew all of it so, my previous thread was not completely solved.
Any help apreciated. Thanks

I would completely backup to an external source and do a clean install. Delete every partition and start over from unallocated drive.