A new feature in Windows XP is the ability to group several instances of the same application as one task icon. This keeps the taskbar from getting all cluttered when working with several applications. It also helps in the situations where Internet Explorer is scripted by a malicious site to start a zillion instances of itself making it easy to close all down. But if used to having 3-4 Internet Explorers or Notepads running at a time without wanting them to be grouped together then one can get annoyed.

To disable the task grouping completely:

1. Right click the Start-button and select "Properties"
2. Select the "Taskbar"-tab
3. Untick "Group similar taskbar buttons"

One can also disable it through the registry with this DWORD value :

[HKEY_CURRENT_USER \Software \Microsoft \Windows \CurrentVersion \Explorer \Advanced]
TaskbarGlomming = 0 (Default = 1)

One can change the behavior of the task grouping :

Group by age(oldest first) = 0
Group by size(largest first) = 1
Group when size 2 = 2
Group when size 3 = 3
Group when size n = n

[HKEY_CURRENT_USER \Software \Microsoft \Windows \CurrentVersion \Explorer \Advanced]
TaskbarGroupSize = 8 (Default = 0)

More Info MS KB281628

Related Configure the taskbar