The disk partitioning utility built into Windows 7 doesn't let you extend partitions to the left, so to enlarge your leftmost partition, you need a free repartitioning program.
Sometimes installing OS X may result in odd behavior if the partition scheme on the drive is not set up properly. If you are experiencing problems, a simple repartition during setup may be an easy fix.
It's possible to keep your working copy of Windows XP and perform a clean install of Windows Vista. Here's how.
Let Ubuntu repartition your hard drive so you can jump between Windows and Linux on the same PC.