What you need to do is find out exactly what operating system you have as Vista. Use the model of the computer and go back to the manufacturer web site and under support get a list of what OS was installed on that machine...

This might help you decide also....

Windows Vista ships in six editions.These are roughly divided into two target markets, consumer and business, with editions varying to cater for specific sub-markets. For consumers, there are four editions, with three available for developed countries. Windows Vista Starter edition is limited to emerging markets. Windows Vista Home Basic is intended for budget users with low needs. Windows Vista Home Premium covers the majority of the consumer market. For businesses, there are two versions. Windows Vista Business is specifically designed for small business, while Windows Vista Enterprise, the premium business edition is only available to customers participating in Microsoft's Software Assurance program. Windows Vista Ultimate contains the complete feature-set and is aimed at enthusiasts.

All editions except Windows Vista Starter support both 32-bit (x86) and 64-bit (x64) processor architectures.

In the European Union, Home Basic N and Business N versions are also available. These come without Windows Media Player, due to EU sanctions brought against Microsoft for violating anti-trust laws. Similar sanctions exist in South Korea.

Dell and Microsoft partnered up to support (PRODUCT) RED. Microsoft released the Windows Vista Ultimate (PRODUCT) RED that exclusively will come together with Dell (PRODUCT) RED Computers.

Once you decide or aquire the correct version then reinstall it over the older one installed and you will still be able to use the COA code on the side of your computer to activate it!

Without the exact system you are kind of stuck because during the precheck if everything doesn't just match perfectly it will lock you out.