Do you really want a popup window or a "bubble box", which is not a new window, but like an extended tooltip. If you can avoid using a popup do so. I turn popups off and won't open them on principle unless it's telling me I've got a new PM on Daniweb!
If you can get away with not having a new window, there are absolutely loads of scripts out there for a js solution. However, people without js won't see the popup (but I would imagine that's a small number nowadays). May be an issue for handhelds though.