I recently added a 3TB HD to a 4-pentium desktop computer running XP because the MB indicated it will support 3TB. I installed the same HD recommeded in the MBO manual. And yes I formatted the HD. However, it takes 10 plus minutes for windows to start up after the upgrade. I checked the ram, then researched the options with RAM, which only increases speeds after windows set up. I repartitioned the HD and resintalled XP to mo avail. What to do? Any suggestions? I'm all ears!

Is this an actual 4 pentium MB or did you mean to say a pentium 4 ?

What you might do is repartion the drive to a smaller segment. Also what drivers are you using with this drive? (SATA or something else)

Some tips:
Windows partition (c: typically) maximal advised 100 GB
Other partitions (d etc) size does not matter but try to avoid more then 1TB (1024GB).
Windows XP 32 bits does not fully support those size`s even though your Mainboard might support it.
RAM size advised of 1GB (1024MB)
Also make a fresh install clear the whole hdd, make and partition for your core files of 100 GB and partition the remaining size of the hdd when windows xp runs.
Use windowskey+r type diskmgmt.msc to create a extended disk and logical partitions.

I believe that not only do you need a motherboard that supports larger than 2TB but an OS designed for it too. That means Vista and above (it might be Win7 and above). If you want to use it as a secondary storage drive just split the partition into 2 volumes using your old drive for the OS. Using it as the boot drive I would split it 1tb and 2tb with the 1 being System OS. Then you should be able to use XP. I have several 2TB externals and a few 1 to 1.5 internals that are recognized fine. Just don't format the drives as fat32 Duh!