The just-released Windows Vista SP1 Release Candidate (RC), like the previous beta, does more to improve the internal plumbing of Windows Vista than it does to make any major changes to the interface. SP1 RC targets performance, reliability, and security, leaving the operating system's features and functionality largely intact, with the exception of allowing users to substitute an alternate search tool for the one built into Vista, and removing the Search link from the Start menu.
When the final version of SP1 (currently slated for next year) is released, the big news will be the death of the so-called Kill Switch, which Microsoft prefers to call "reduced functionality mode." Whatever you call it, though, the elimination of the switch will be good news. Currently, if you don't activate your retail copy of Windows Vista after 30 days, your desktop turns black, and your icons and the Start menu vanish. You can't open your files (although you can copy them). You're able to use a Web browser for only an hour before you get logged off.
The same thing happens if Windows Vista decides that you've installed it on a different PC than your original one, and you ignore a three-day grace period for contacting Microsoft. Vista might also decide it's been installed on a different PC than the original if you make a substantial number of hardware changes to your original PC.
In the shipping version of SP1 (though not in the current RC version), the "Kill Switch" will become more of a "Nudge Switch." You'll be frequently reminded that you need to activate Windows, and the desktop background will turn black. Try to change it to another background, and an hour later Windows will turn it black again. In addition, you won't be able to download signed drivers and optional updates via Windows Updates, although you'll still be able to get critical security updates. Not a pleasant experience, certainly, but at least you'll still be able to use the operating system.
Changes since the previous beta
Not much has changed in the RC version of SP1 compared to the previous beta, although there are slight differences. Installation on my 1.83-GHz Core Duo laptop took under an hour; the previous beta took an hour and fifteen minutes. The previous beta didn't clean up after itself, and left about 1 GB worth of files in a folder that was no longer needed after installation. RC, on the other hand, deletes that directory and files.
A bug I found in the previous beta still remains: On my home network, which uses a Linksys WRT54GX4 router, I couldn't connect from my SP1 RC test machine to any other Vista PCs on my network. And I couldn't connect from Vista PCs to the test machine, either. However, I could connect to XP machines, and from XP machines to my test machine. That bug appears to be an isolated one, because other reviewers have been able to connect to Vista PCs on their networks.