Okay so I bought a new computer recently that uses Vista, and enclosed my old computer's hard drive and the enclosure said that it was Vista compatible but I installed the driver but it doesn't appear when I try to access it. Can someone help plz?

Have you looked in Disk Management for the hard drive? Go to Start and type in "diskmgmt.msc" (without the quotes) and press Enter.
Have you looked in Device Manager to make sure that the USB (I presume) device is recognized? Go to Start and type in "devmgmt.msc" (without the quotes) and press Enter.

Are you sure that the enclosure has enough power to run the drive? If it's not a laptop drive, you'll need an external power source to make it function (and some laptop drives will require this also - generally any that require more than 500 mA of current).

Are you sure that the drive is OK?

Have you switched the jumpers on the drive (if it's an IDE drive, SATA drive jumpers won't need to be moved)?

I checked Dev. Mgt and it did appear under the USB but it still doesnt show up
It's a laptop drive and the enclosure came with a cord for power to run to it.
The drive should be okay. It had a Start-up error but I've had one do that before and it still worked once enclosed

Check in Disk Management - go to Start and type in "diskmgmt.msc" (without the quotes) and press Enter. Scroll down to the device and see if it's got the same drive letter as another device - if so, change it.

It doesn't appear when I run that :/