Mark Russinovich is one of the most well-known and respected gurus in the IT industry - his book co-written with David Solomon, Windows Internals, is considered the "bible" of many involved in working with very detailed, low-level features of Windows (NT upwards).
He also has the website Sysinternals which has a collection of very useful utilities (most free) for examining the inner-workings of Windows including a souped-up Task Manager (Process Explorer), Rootkit Revealer, tools for logging registry and file access in realtime (RegMon & FileMon), and many others.
Most blogs are uninteresting to me, but Mark's is one that I follow with keen interest:
http://www.sysinternals.com/Blog/
Entries so far from this year:
2006-01-03 The Antispyware Conspiracy (especially interesting if you have used the "SpySheriff" product)
2006-01-15 Rootkits in Commercial Software (Sony aren't the only ones)
2006-01-18 Inside The WMF Backdoor (addresses Steve Gibson's claim that Microsoft put a backdoor into the WMF handling code on purpose)
2006-02-06 Using Rootkits To Defeat Digital Rights Management (shows how Alcohol and Daemon Tools use rootkit methods)
2006-03-02 Running As Limited User - The Easy Way (a guide on running apps as a limited user even when logged in as Administrator)
The blog has an RSS feed as you would expect.
If you are interested there are also documents if you follow the Information link, with more very low-level guides, a walk of the XP source code tree, errata for Windows Internals and much more.