No disadvantages that I can think of.
OEM hard drives have the same warranty that comes with a retail hard drive. Retail hard drives come in a pretty box with a diagnostic disk, four mounting screws, parrallel or serial IDE cable and installation instructions.
The diagnostic software and installation instructions can be downloaded and viewed from the hard drive manufacture's website.
An OEM version of Windows is exactly the same as the retail version except you do not get free end user support from Microsoft and you don't get a pretty retail box.
It is not worth paying a $100 or more for the retail version of WinXP because all you are getting is free e-mail and telephone support for the product. If you have trouble with WinXP you still can use the Microsoft troubleshooting database and the thousands of other webpages available to help you with any problems. Also with the OEM version you still get to download Service Packs, Critical Updates, Regular Updates and Driver Updates for free from Microsoft.
If you are installing WinXP on the new hard drive use the WinXP CD-ROM to format the disk. If you are installing the hard drive as a second drive in a system with Windows already installed on the primary hard drive use diskmgmt.msc to format the new disk.
Not always true, OEM white market drives have full warranties, but OEM grey market drives have no warranty by the manufacturer. While companies such as Newegg get their drives from the manufacturer (white market), many smaller companies get left over drives from major OEM's (grey market). And the major OEM's get a huge discount on the drive by purchasing it WITHOUT warranty.
<font color=blue>Only a place as big as the internet could be home to a hero as big as Crashman!</font color=blue>
<font color=red>Only a place as big as the internet could be home to an ego as large as Crashman's!</font color=red>