Practices/tools to remove personal data from a computer

"If the company has taken any backups or you have copies on other machines, you may be in trouble.

If you are only concerned with files on just your machine, and you have the ability to install programs, I would take a look at Fileshredder, it should be able to delete the files and make them non-recoverable.

Sysinternal also do a command line tool, called sdelete"

"I'd usually recommend DBAN, but since it's only for a single directory, you can use Sysinternals SDelete:

SDelete is a secure delete application
that can overwrite a disk's unused or
unallocated space, or securely delete
existing files. It implements the US
Department of Defense clearing
standard DOD 5220.22-M, and you can
specify the number of overwrite passes
it makes. It can also wipe Windows
NT/2K compressed, encrypted and sparse
files (it uses Windows defragmentation
API to accomplish this), and clears
the NTFS Master File Table (MFT). (It
does not delete the filenames of your
files in the free disk space though -
apparently it is not possible to do
this in NT/2K.) This program comes
with source code and works under
Windows 95, 98, NT 4.0 and Windows
2000. It is a command line utility.

there is also FreeEraser for a GUI approach."