iPhone is not supported on either XP or Vista IF the OS is 64 bit. In other words, you won't be able to connect from iTunes to iPhone on either Operating System; 32 bit is fine and MOST people have 32 bit
I've had my iPhone all of 6 days. I have Windows XP Pro SP2 with MS Office 2003. The installation is about a year old since I'm a heavy user and do a clean install every couple of years or so. To date, I've had no problems with iTunes for Windows or syncing the iPhone to Windows. In fact I love iTunes. It is a different animal than it was when I looked at it several years ago. I have made it my default player. I updated to iPhone software version 1.1.2 yesterday and all went smoothly. So from my point of view the iPhone with Windows works just fine.
I am running Vista Ultimate 32 bit and have no problems at all with my iPhone. I have unlocked phones (1.0.2 and 1.1.1) and everything via Vista. Just make sure you do a reboot before you try anything and ensure your firewall (if any) are turned off.
I buggered up my phone the other day and did a huge baseboard downgrade, fix and upgrade back to 1.1.1 using Vista. It didn't give me any problems at all. Just make sure you disable any firewalls & screensavers and you should be fine. Always best to start any modding with a fresh reboot also.