I reinstalled Windows XP Home and my computer no longer seems to recognize my USB 2.0 four-port hub (internal) from Adaptec.

I've tried to physically reinstall the card for plug-and-play and install the latest drivers from Adaptec but this didn't work (although I unpacked the files and it didn't seem like much of an install, anyways).

The frustrating thing is, when I turn on my equipment I get a message saying that there is a malfunction, and that a device has exceeded its power on such and such port. And it says to fix click the balloon...

Under USB Root Hub (5 ports)

Unused port
Unused port
? Unknown USB Device
Unused port
Unused port

The options are to click Reset or Close... reset does nothing.

What I can't understand is why this yellow question mark message doesn't appear anywhere in System > Hardware > Device Manager

Is this an XP thing? Any help is greatly appreciated.

You need Service Pack 2 installed, the Question mark is because the USB Driver is not installed and will not be until you update to SP2.

Whenever you install Windows from scratch, you must install all the drivers for your mboard hardware, especially the main chipset drivers, after Setup has finished so that Windows has all the proper drivers for and information about your mboard components. If you have the CD that came with the mboard, use that. If you don't have the mboard CD use the drivers available in the software downloads for your mboard model or brand name system model. In that case you may need to get the main chipset drivers from the maker of the main chipset's web site - they are often not on the mboard manufacturer's or brand name system's web site.
If your XP CD does not have at least SP1 updates built into it, USB 2.0 controllers cannot be recognized. They will be recognized once you install SP2 updates.

If SP1 ot later updates ARE included on your XP CD, or you have installed SP2 updates if they weren't, and you have all the drivers for the mboard loaded, THEN if the onboard USB 2.0 still does not work you may need to find drivers.
Make sure the USB 2.0 is enabled in the bios Setup.

If you happen to have a VIA based motherboard you need to run the 4 in 1 Hyperion chipset driver.

Hmmm, ok thanks. Using CPU-Z

CPU Page
My processor is Intel Pentium 4 630

Mainboard Page
Chipset Intel i945G/GZ
Southbridge Intel 82801GB(ICH7)

... and in Device Manager, it looks as though all that is updated under System Devices. I went to Intel's site and upgraded this when I saw the Yellow warnings...

So I thought that would be it.

Your post's last statement is as clear as mud, and you have not commented on anything I said in response 2.
Do all that is applicable in response 2.

Have you installed all the drivers for the mboard, preferably from the CD that came with the mboard?
Does your XP CD include SP1 or SP2 updates?
(it's printed on the CD).