While you're at it, did you know Windows XP already comes with Windows Messenger?
1. Click Start
2. Select "All Programs"
3. Select "Windows Messenger"
4. Login using your MSN account.
This is free and already pre-installed on your computer, so it is good for testing things out too. You can create two MSN accounts (using the links in my FAQ above), one for your computer, one for your BlackBerry. And do some tests sending messages back and fourth between the BlackBerry and the desktop computer -- at least while you're still within your 2 week VeriChat trial period.
You probably do not need IM anyway, but if you are going to play with it anyway -- you might as well do the simple 1-2-3-4 steps above too