Having just installed a hard disk someone gave me into my newly built puter - (got given a lot of the parts, so only paid for mb/chip, monitor, video card and extra ram), found when I installed it, it was picked up in bios, device manager but not under computer. This was because it hadn't initialized under windows - but was visible.
To get round this, do the following:-
Start, run, diskmgmt.msc - hit enter.
Search for your new disk on the list, right click on the bit to the left of your new disk name, click on simple volume and a wizard starts. You then get to choose a drive letter (E: for example) and choose a filesystem (NTFS or FAT32) (I went with NTFS as using a 64bit OS)
Click on finish and a few seconds later, it'll appear under your computer (whereas before, it wasn't visible). You can check in Device Manager, right click, properties, volumes and click on the populate button on the right hand side).
Hope this helps someone out - only takes a few mins, is easy enough and stops you pulling your hair out, or searching through google!!
Wurzel