GIBSON RESEARCH CORPORATION
Excellent plce, here you can test your firewall using 'Shields Up' (a free service), you can also purchace the excellent diskrepair utility Spinrite plus a whole lot more.

FRISK SOFTWARE
F-Prot is a neat FREE anti-virus utility (MS-Dos based), absolutely worth a peek.

ZONEALARM
ZoneAlarm is a must for personal internet protection, it is what you call a personal firewall which is a pice of software that will help stop intruders gaining access to your computer.  ZoneAlarm is FREE for personal non commercial usage.

PACKET STORM
Packet Storm is a great ressource of security related content.  This is however mostly aimed at professional users.

RAW LOGIC - NET VIEW
RawLogic offers some usefull utils to probe your ports for security holes.  This is mostly aimed at professional users.

ANNOYANCES.ORG
A place with various tips and tricks on Win NT / 98 / 95.

L0PHT INDUSTRES
A 'company' that has speciallized in Windows NT/2000 and its security problems.  Administrators might find quite a few usefull utilities here.

GEORGI GUNINSKI SECURITY RESEARCH
This guy is aimed at finding all the security holes in Microsofts products, and he does a good job of it! Check this web-site to find information on the current security problems with IE and the likes.

WINDOWSBLINDS.NET
Modify your windows layout, make your desktop look like OS2 or Linux.  Not very usefull but cute.