I use a tool to easy switch it on and off.
Once installed, right klick on the tray symbol, go to "Direct3d =>Antialiasing" and select 4x.
You can get "Ati tray tool" here: (homepage)
http://www.radeon2.ru/atitray/
In general i´m not a fan of tray applications, but this tool is a must for
ati-users. I would call it stable and there should be no problem on de-installing if you don´t like it (you will like it, i promise)
After installation there will be an fps display in the top right corner of the screen if you start a game or play video. Good for testing, but if you want to switch it off (Neverwinternights f.e. doesn´t like it) go to "tools and options => OSD and remove the first marker"
The tool has also options for overclocking your card. Please don´t do it!! Great chance of wrecking your hardware. During installation you will be asked to exclude overclocking options. Just say yes here.
If you are strict against such tools (like i was before)
you can switch AA in your ati driver settings. Hard to explain for me , cause i use a german win2k, and you prob. an english winXP. My bad english doesn´t make it easier. I´ll try:
1. Right klick on your desktop
2. A menue appears, left klick on properties
3. A window appears, you know it, you can change screensaver and desktop stuff here.
4. In the top right corner click on settings
5. In the low right corner click on additional settings
6. A new window appears, click on "3D" at the top right corner
7. I´m stuck now, don´t know how to explain further, my english is too bad. Perhaps you use the net-framework version of the ati-driver (there a 2 versions of the newer ati drivers, one with net and one without) and i don´t .
I suggest to use the tool, it´s safe and easy to use.