Well, the problem wiht the slimline pavilions is that they are a tiny box with as much computer equipment as possible crammed into as small a space as possible. If you are keeping the existing case, then your only option is going to be the stock heatsync, or an external water cooling setup. Any other cooler on the planet probably isn't going to fit. If you are putting the motherboard into a different larger case, then you have a range of choices, but you probably can't, since most of those were either proprietary or BTX, which might as well be proprietary anymore.
If they are encountering overheating issues, I'd reccommend a new case ($79ish, might also need a PSU at $80-ish) and a new motherboard ($99-150, depending on features) and then just move everything over rather than trying to make a bad design work when it isn't.
An external ccooling system, like a Koolance Exos unit will run around $300, and will require routing tubes out the back of the PC. Thats also completely overkill for that particular computer.
If you can get me an exact case model number and dimensions of the processor fan area, I can see if I cna find something that will fit, but chances are you are probably out of luck.