Many people know that you can use the Ctrl + Enter keys to add the "http://www." to the beginning and ".com" to the end of a web site name in Internet Explorer and hence save a little bit of typing. Look at page 22 of the current Windows Vista Official Magazine (UK issue 4) if you need to check it out.
But what if you don't mostly go to "dot com" websites?
Easy, just change the quick complete entry for Internet explorer to reflect where the domains you most frequently visit are registered.
Method 1
From the Start menu, find "run.." and enter regedit (ok).
Find and highlight HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Toolbar in the left pane.
From the edit menu select new ..key and type QuickComplete (enter).
The new key appears in the left pane. Highlight it and select edit.. new.. string value.
A box appears in the right pane names "new value #1" - overtype the name with QuickComplete (enter).
Right-click the new value and modify the value data to show http://www.%s.extension , but don't type extension, type your preferred website registration indicator such as .co.uk or .org - see the image below for the entry you should now have in registry Editor. You can see the full key name in the lower text bar - just!

Method 2
Open Notepad and type or copy and paste the following text:
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Toolbar\QuickComplete]
"QuickComplete"="
http://www.%s.co.uk
"
Save this as CtrlEnter.reg ensure the drop down says all files not default text document.
Double click the resulting file in explorer to import the new settings into the registry.
You will have to okay both the UAC and Registry Editor warning dialog boxes. I created two reg files, one to set Ctrl+Enter to ".co.uk" and another to put the shortcut back to ".com"
Warning!
If you've never delved into editting the registry, this is probably one of the easiest and safest changes to make, as long as you double check everything on screen matches the above. Remember, never make changes to your registry unless you're willing to suffer potential consequences and always have backups or System Restore points available in case something goes wrong - its your responsibility, not mine.
This edit works in both XP and Vista (32 bit versions). The registry keys are the same in the 64bit versions but you must ensure you access the correct registry editor - see the first screen of the registry editor help files for detals.