Do you have the disks that came with the computer - either the OEM Windows
Vista CD, or a Recovery disk (note this would return the system to "factory
condition" - else it might have a "recovery partition".
You don't give any clues as to the type of computer etc.....can you give
more details?
First thing would be to contact the support for your particular brand of
computer, asking "how to reinstall windows". Microsoft can't help if the
Windows is OEM - the "Original Equipment Manufacturer (OEM)" is responsible
for support for the hardware it sells (including any bundled software), or
technically, the retail vendor could also provide support for the computer
and hardware.
Read the manual that came with the computer - it would likely tell you how
to run the recovery or reinstall routine.
Also can you give us some idea of the action you took (or the problems you
were having) that "messed up" the computer? A description of the problem
might lead the way to a simpler fix than reinstalling Windows.