How to reinstall Vista from Recovery Drive

Hi, I have the problem with Vista where Updates are downloaded but then will
not install etc. I do NOT have a Vista CD as Vista was preinstalled on a Dell
540 Desktop. I do have the OEM number as shown in system files. I want to be
able to reinstall Vista and hope that will fix the problem - but I don't want
to have to reinstall any programs or lose any files already on the computer.
Can anyone advise how to re-install Vista from the Recovery Drive or
elesehwre on my computer?
Thanks. Neil