Well, updates can be downloaded separately from Microsoft.com if you know their Knowledge Base number (KB no.) but its very tedious and moreover you cannot make sure which updates are required/appropriate for your system. Windows Update Client does the job of finding out what updates apply to your system and provides you with an interface to download and install them.
Now since using the Update Client is not an option, Service Pack 1 is the option you should go with. Service Packs contains all the previously released updates and patches (those released before the Service Pack itself, ofcourse
). Although its not sufficient to do so, but its the least you should do. SP1 for Vista is a must have upgrade. Install it ASAP.
Frankly, relying on SPs for updates is not a recommended practice as the time frame between two releases of SPs for an OS os quiet large, and you remain unprotected from the vulnerabilities found during that period, which can be a lil dangerous as most of the times, these vulnerabilities are being actively used by malwares at the time of patch release.
Windows Vista Updates Problem
Linear Mode
