You can use different fonts in Sigil. The fonts folder in Sigil has nothing to do with the fonts folder of Vista. It is a folder within the ePUB.
In short, the procedure is as follows:
1. Upload the font you need in Sigil. It will appear in the fonts folder in Sigil.
2. Create in your stylesheet the definitions of the font via the @font-face class
3. Define in your stylesheet that the font is used for example in headers
Step 1 is simple. The other steps are defined at various steps here already if you search. A nice description is at the site of Liz Castro
You can do almost everything within Sigil with regards to fonts and styles via stylesheets. However, you must use Code View and do it yourself via CSS and XHTML. Bookview is very limited in functionality and will not allow you to change fonts and alike.