Short story: Forget about it. Long story: You can still forget about it, but Adobe is sorry for you.

"As many have noticed, the Lightroom 4 public beta we released last week does not support Windows XP.  This decision did not come lightly and

was based on a number of factors.  With each version of Lightroom, our goal is always to provide a consistently excellent customer experience.  Developing and testing across Operating System versions and platforms to ensure that we’re achieving this goal is a significant effort that takes time and resources.  XP is substantially different from Windows Vista and Windows 7, and requires a independent testing matrix, which increases the complexity of our development efforts.   Every cycle we need to make difficult resource tradeoff decisions and operating system support competes heavily with new features and performance enhancements planned for each version.  The decision this cycle was to end XP support for Lightroom going forward.