Here's a step by step guide to upgrading from XP to Vista. If the upgrade
still fails it is likely some hardware is not Vista compatible or you
still have some software running that is not Vista compatible.
1) Backup your PC.
2) Back it up again.
3) Test your backups.
If you skip the above steps please don't whine that the upgrade trashed
all your files. Any process that involves this many changes to the file
system is fraught with danger. If you don't have a full backup of your pc
before starting the upgrade you are an idiot and shouldn't really have a
pc to start with

This may sound harsh but it is reality.
4) Run the latest version of the Upgrade Advisor and note anything it
flags. 5) Uninstall (not disable) all antivirus, antispyware, firewall,
disk utility, and system utility programs even if the upgrade advisor
doesn't mention them. You will need to install Vista compatible versions
after the upgrade is finished.
6) Uninstall (not disable) all programs that the upgrade advisor flags as
possible problems. You will need to install Vista compatible versions
after the upgrade is finished.
7) If possible remove all hardware that the upgrade advisor flags as
incompatible.
8) Make sure you have Vista compatible drivers and software for all your
hardware devices burned to CD. Don't just look for drivers that the
upgrade advisor mentioned. If possible have drivers ready for everything.
9) Physically unplug any external devices like portable hard drives,
printers, card readers, flash drives, cameras, etc..
10) Run a chkdsk on all the partitions on all hard drives still connected.
11) Defrag the system and boot partitions.
12) Start the Vista upgrade process.