From Download Squad:

Aside from support for Windows XP, Windows Search 4.0 has a few other significant updates:

• A number of bug fixes
• Get your results 33% faster
• Remote Index Discovery now works with every version of Windows, meaning you can index files on any Windows machine on your network
• If an error occurs, Windows Search will rollback to your last known working state rather than starting the entire indexing process over again from scratch
• Encrypted files can now be indexed