The HP HD100 is currently on sale now at newegg:
http://www.newegg.com/Product/Product.asp?Item=N82E16827140022
HP 2.4X HD DVD-ROM, 5X DVD-ROM, 14X CD-ROM USB 2.0 External HD DVD-ROM reader Model HD100 - Retail
Unfortunately, it is $429.99 now and $379.99 after $50 rebate. Its probably basically the same as the Xbox HD DVD (ROM) drive but cost about twice as much. The HP HD100 does come with PowerDVD 6.5 HD DVD Edition.
PowerDVD 6.5 isn’t likely Vista compliant and is an earlier implementation but it should have some functionality in XP
PowerDVD (7.1-7.2) Ultra is a newer implementation and should be Vista compliant.
Given the price of the HP HD100 it would actually be cheaper to buy the Xbox HD DVD drive and Cyberlink PowerDVD Ultra by aproximitly ~$100 and PowerDVD Ultra should also assure better Vista support.