Error crops up after weeks of running a website

I have a website based on Vs2008, .NET 3.5SP1 C# code. One of the functions requires it to interop with a COM-based DLL that creates PDFs.

About once every 3 to 4 weeks, anything COM related (e.g. generating PDFs) is completely hosed. I get the following error:

Creating an instance 开发者_如何学编程of the COM component with CLSID {7A91D523-4F04-42BD-8441-4C15B9D7C04F} from the IClassFactory failed due to the following error: 800401f7.

I looked up 800401f7 and it means Some error in application program.

If I do iisreset, everything is back to normal for a few weeks.

Why am I getting this error and what does it really mean?