Help! Some websites don't want to show up... but others work fine. What's going on?

This has been a week of breakthroughs for me in the world of IT odd-problem solving.

For starters, I came across the massively useful OutlookTools, which sorted out some long-standing Outlook issues for me.
Another killer has been a problem involving some websites which will simply fail to load, or take extremely long to load, in an SBS 2003 Domain environment.
This while other sites load without any hitch or headache... weird...

When I first came across this problem, the first thought that jumped into my noggin was "DNS"... must be DNS.
This kind of problem just reeks of an incorrectly configured DNS, and so my fruitless quest began to reconfigure DNS settings, double check Firewall rules, double check IP / Subnet Mask settings, clear browser caches, reset browser settings to default, disable antivirus software... you name it I did it.

That is, until yesterday when I realised (through much testing), that the problem lay not on the SBS 2003 Server, but rather on the workstation in question.
A little setting called "MTU" turned out to the source of all my frustration. MTU, for those who are now tilting their heads slightly to the left in confusion, is the size (in bytes) of the largest protocol data unit that the layer can pass onwards.

Umm.. OK...?
Well in layman's terms it's a network setting that can either help you, or hinder you depending on the type of internet connection you use and your network environment.
For ADSL (in my case), the ideal setting should be 1492, and not the Windows XP default of 1500.

So without getting too technical, the quick way to fix this was to head over to Speedguide.net, and download a program called TCP Optimizer, which does just what its' name would suggest.
I ran TCP Optimizer, selected the relevant network connection, selected "Optimal Settings", and then (this is the important bit), I selected "PPPoE DSL", and I clicked on "Apply Settings".

After confirming the changes, TCP Optimizer asked if I wanted to reboot to apply the settings, to which I said Yes Please, and so the problem made its' way out of my "headaches" tray, and into my "solved" tray :)

YAY!
Hope this works for you as well :)