When using VISTA make sure you run atitool with admin rights.
Right click atitool icon and select run as admin. I use the 0.27 v atitools on Vista home premium with no problems.
If atitool still does not work and you just want a simple overclock on your card use the ATI cataylst.
1. Load up ATI's Cataylst Control Centre, part of its driver package, by right clicking on the desktop and selecting its name on the menu that pops up. When prompted, choose Advanced rather than Basic mode.
2. From the list of settings on the left, you want the bottom one - ATI Overdrive. If you're sure you want to do this thing, you then need to click on the padlock icon over on the top right to allow speed changes.
3. The best option if you're a first- or indeed a fortieth-timer at graphics card overclocking is to click Run Automated Clock Configuration Utility. This will test the card's GPU and RAM at various different speeds, working out what's safe to run at.
4. Once it's done you should notice a few more megahertz from the default settings. Click apply, then gun up a few games and give them a quick run through to see if the new speed is stable.
5. If your new setting isn't causing glitches or overheating, you can check 'Preserve ATI Overdrive Settings at logon' so the card will remember its new speed. If the new speed did cause some problems, try lowering the sliders next to Graphics Processor and Graphics Memory Status by, say, 10Mhz at a time.