Multiple instances of Yahoo! Messenger

If you want multiple instances of Yahoo! Messenger in Windows, then just add to registry the following key:
[HKEY_CURRENT_USER\Software\yahoo\pager\Test]
"Plural"=dword:00000001

By the way: you can download the Yahoo! Messenger client from here.

Step by step:

1. Open Start -> Run and type regedit, then click OK
2. Click the [+] sign beside My Computer
3. Locate HKEY_CURRENT_USER and click the [+] sign beside
4. Inside the opened tree click the [+] for Software, then Yahoo, then pager
5. Finally click on Test
6. Make sure you have the Test key selected in the left panel or click on it to select
   
7. In the right panel you should see only a default value: (Default) REG_SZ (value not set)
8. Right click on this right panel and choose New and then DWORD Value
9. A new key is created with the name New Value #1; change this name to Plural and hit ENTER
   
10. Right click on Plural value you've just created and choose Modify
11. Change Value data from 0 to 1 and click OK
12. Close Regedit window and start as many yahoo messenger clients as you like.

Note: Although you can use multiple instances now, you can't log in more than once in the same account.