These conversions will vary by file type. An image will store metadata differently than a document, which will store metadata differently than a video. Even different files types of similar things (for instance, BMP and JPG) store their metadata differently.
As such, you'll need unique applications for each of these.
For images you should be able to use IrfanView, which will be handy because it can batch process large numbers of files easily.
For documents, MS Office has certain capabilities or processes, as defined by MS, how to minimize metadata included in the documents. Unfortunately, the MS documentation mostly deals with old versions.
For versions 2003 there is a MS tool for removing the metadata: http://support.microsoft.com/default.aspx?kbid=834427
For pre-2003 versions MS has documents on how to remove this info: http://office.microsoft.com/en-us/excel-help/find-and-remove-metadata-hidden-information-in-your-legal-documents-HA001077646.aspx
For new versions of office, Payne Consulting Group has a cleaner (I worked at a large lawfirm that used this for Office 2010) but I'd imagine it's horribly expensive. There is also iScrub and another cleaner documented in a Tech Republic article: http://www.techrepublic.com/article/keep-microsoft-office-documents-clean-with-iscrub/5035132
I found a MS article about inspecting and cleaning documents without using any other tools. It's manual, but it's included in the MS products already, and the solution works in version 2010: http://office.microsoft.com/en-us/word-help/remove-hidden-data-and-personal-information-by-inspecting-documents-HA010354329.aspx