It depends on which SharePoint but the normal method would be to use the old system's databases when installing the new system.
There will of course be NO link between the two systems after that. All you will have is a full copy of your existing system at the time the databases were used. Changes made to one will not go to the other.
WSS FAQ sites:
http://wssv2faq.mindsharp.com and
http://wssv3faq.mindsharp.com Total list of WSS 3.0 / MOSS 2007 Books (including foreign language)
http://wssv3faq.mindsharp.com/Lists/v3%20WSS%20FAQ/V%20Books.aspx