Go, Shawty, it's your birthday. We're gonna party like it's your birthday. We're gonna sip Bacardi like it's your birthday. But do you know how we add this f*cking smiley? 
Since MSN Messenger 6.2 your shortcut codes are automatically replaced with corresponding emoticons while you're typing. Because of this new feature, users can not use the <:o) shortcut for Party smile anymore because it will get replaced with the surprised smiley :o). Here's how to add it to your conversation:
a) You will have to select it from the emoticons shortcut list (in MSN Messenger 7, you can Pin it so the smiley always appears in the list)
b) Or type <o) and then add the : before the o.
Party on!