It might be your USB-to-RS232 adapter.
There is a known issue with USB-to-RS232 adapters that use the Prolific chip. There are counterfeit chips of poor quality being incorporated into some adapters. The latest official Prolific drivers (e.g. Win7) will not work with the counterfeit chips. Prolific has a tool to detect the incompatible parts that makes it easier to identify the problem. You can look at the Win7 Device Manager for a yellow mark on the USB adapter icon. Here's the page with the tool, the driver, recommended cable brands, and information on this problem: