Originally posted by Kyle Fenton
Software, especially games and older software, will have to be re-writen. Hardware may also be affected, especially if the hardware you have is no longer supported by their manufacture. Microsoft even said, that if you machine orginally came with Windows 98 or 95 don't expect it to run on Windows XP.
While it is true that XP is based more on the NT/2000 Kernel than the 9x kernel(grouping ME into this), and NT/2000 has had compatibility issues with some software, Microsoft is making this one of their main focus points, knowing that XP will have more of a home user presence than 2000 did, and making compatibility with existing software, especially games, one of their important goals for the XP release. However, due to the different architecture of the NT/2000/XP kernel, there will be some software that either will have to be rewritten or simply just won't work, mainly those programs that used 9x specific code to work their magic.
As far as the hardware compatibility issue, my understanding as to the "if your machine has 95 or 98 on it, don't automatically assume XP will work" statement is not that they are dropping compatibility for older hardware(note: it's reported that you can use Win2000 drivers with XP if that's all you can get for a device,) but it's rather due to the increased requirements of the XP kernel. Since it's based on NT, this comes as no real surprise, as NT has always needed a more powerful system than 9x. For example, here's the list of requirements for the XP RC1 Preview release:
-Minimum 200Mhz processor, 300Mhz recommended (Intel or AMD recommended)
-64MB RAM Minimum, 128 MB Recommended
-1.5 GB free disk space(maximum, depends on installed options)
-SVGA 800x600 or higher
-CD-ROM or DVD Drive
-MS Mouse or compatible