In-Depth Windows Vista Beta 2 Preview by Anandtech
 

For those systems that can't offer any real GPU acceleration at all, Vista is capable of also dropping back down one to two interfaces that use just the GDI+ functionality, and are intended to replicate XP and 2000 abilities respectively. Basic is intended to be the XP-like rendering mode for Vista, and while it currently uses a Vista-styled theme, this theme is going to be replaced for the shipping version. Classic will be the 2000-like mode using the 2000 interface style and rendering features. Because neither of these modes use the compositing engine, they drop the GPU requirements significantly (down to practically those of Windows XP/2000), but in the process lose the benefits of using a fully accelerated desktop rendering system and bring back the drawbacks of GDI+, such as higher CPU usage in some situations.