If you are after perfect conversion then there is no real solution available / viable other than somehow involving Office. Plain MS-Interop is a nightmare and very unreliable, but there are good third party solutions available.
Give the Muhimbi PDF Converter Services a look. It installs in your environment as a scalable and robust Windows Service and has specifically been designed for use from server based applications such as ASP.NET.
It comes with a friendly web services based interface that allows it to be used from most modern environments such as Java and .NET. It supports all common as well as some not so common file formats. Watermarking and PDF Security is included as well. If you have SharePoint in your environment then a SharePoint optimised version is available as well.
Disclaimer, I have worked on this product so the usual disclaimers apply. Having said that, it works great.