virtualbox.org

Virtualbox version - 4.3.0 r89960 with guest additions installed
Host OS - Windows 7 Enterprise 64-bit 4GB Ram
Guest OS - Windows 7 Enterprise 64-bit 1.5GB Ram


When running Guest OS, Firefox (24) or Internet Explorer (10) on the Host OS begin consuming >400MB (Firefox using in-excess of 500MB) with only a few tabs open and static data and bring PC to a crawl. When the Guest OS and Host browsers are closed, and only Host browsers reopened, Browsers return to their normal memory usage (~100MB) with the same static data and PC will run smoothly. Updated flash and Java but still running into the issue.

You would be the first one reporting Interferences between Virtualbox and browsers, and would be very unlikely that such interference occur without more people noticing - it's a typical use case!

On the other hand, you are using a lot of free RAM from your host, leaving not much at all to play around with, especially with modern browser, using easily hundreads of megabytes in memory.
500 MB doesn't surprise me (even tho I find it shocking to use so much for web browsing).

Reduce the VM RAM to 1GB and see if it gets better.