I would suggest you to download a standalone pack of Service Pack 2 and install it in Safe Mode.
Refer the links below for more information on Service Pack.
How to obtain the latest Windows Vista service pack
http://support.microsoft.com/kb/935791/
Depending on whether you are running the 32-bit or the 64-bit version of WindowsVista, you may download the Pack from the links given below.
To determine whether you have the 32-bit or the 64-bit version of WindowsVista, click the Start button, right-click Computer, and then click Properties.
If you are running on a 32 bit Vista edition, you may download it from
http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=891ab806-2431-4d00-afa3-99ff6f22448d
If the Operating System is 64 bit, download the pack from
http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=8ad69826-03d4-488c-8f26-074800c55bc3
Regards