I bought one some years back (USB) and it needed WinXP drivers but Win7 just picks it up and autoinstalls. Compared to today's dongles it's pretty bulky, has a floppy antenna and protrudes about 6cm from the USB port.
Probably not the best way to do things but there are nano sized USB ones now or PCIe cards, though I wouldn't waste a PCIe slot on a BT adaptor although there are combo cards that also have Wifi which would make it worthwhile if you want both and don't mind dedicating a slot to it.
Another alternative is one that attaches to an internal USB header on the motherboard then mounts to somewhere convenient such as spare expansion slot cover.
Yet another option though not sure how common could be a 3.5" multi card reader with BT adaptor included. Supposedly this Xeon machine I've got has one but I can't find driver support.
The other consideration is later BT version support. We're up to 4.x now though some of the improvements relate to just using BT to negotiate an 802.11 quick connection, but for whatever device you get probably worth throwing a couple of extra bucks at for a later version.
Seems the nano adaptors are cheap enough that you could just include them in Cornflakes packets so it's not going to be a disaster if you pick a dud.