Intermittent connection problems to Airport Express...

I have weird, intermittent connection problems from my PC laptop (Windows XP Pro) to the Airport Express connected to my stereo. Sometimes my laptop sees the Airport Express, sometimes it does not. It always sees the base station (meaning, I never have problems with an internet connection). Another software program (Airfoil) I use reports this as an “RTSP error,” but sometimes I can connect using Airfoil even though I cannot connect using iTunes. This is not a VPN issue. I thought it might be a firewall issue, but I’ve opened the ports Apple suggests. I’ve also tried different sequences like rebooting the Airport, then rebooting the laptop. Vice versa. Nothing seems to work consistently. Could it be a dynamic IP issue? FYI – I am a total computer novice, so any help would be very much appreciated!