Hi mate,

How does your cable connection connect to your PC - is it with a router?

 

You would need a wireless router. Your cable modem would plug into the router and the router would be the wireless access point for your wifi.
Your PC would connect to the router either with your PCI wifi card or with a network cable.

You would set up your Desire wifi (and any other wireless devices you have now or in the future) to connect to the AP (router).

You can buy a wireless router quite cheaply nowadays (around

 

You can do what you are thinking, by putting your WiFi card into AP mode(Access Point mode), most cards install an app that sits in your system tray, or sometimes just in your start menu/programs.

If you have an app see if there's an option to switch to AP mode, and that will allow you to set up your PC as a wireless access point(remember to set up security on your access point)