Hi Chris and welcome to the PM community,
It's not uncommon for people to fall asleep when they're first starting out with meditation. We often come to meditation because we are stressed out and looking for relaxation, and meditation is so very effective at relaxation that the body naturally takes the opportunity to go all the way into sleep to help regenerate. With practice, the body's need for sleep will lessen and the meditation practice will become more stable.
There are different meditation techniques out there and you've already found Guided meditations, which can be good for helping to relax. The free Discover Meditation course you can download from here is a different type of meditation, essentially the same as what you'd learn if you went and paid a fortune to learn Transcendental Mediation, but without all the ceremony (and religious undertones, though they deny that part) that they incorporate into their 'initiation' into meditation. Michael's CD just teaches the meditation technique itself, which is all that is really needed. This type of mditation is called mantra meditation and is usually done in silence (i.e. without music playing or someone guiding you, though we can't account for background noises

) It essentially involves repeating a mantra (word/sound) over and over and initially putting the focus on that mantra, but then letting it repeat in it's own way. This helps to take you away from thoughts that are arising in your mind, but inevitably at first, you will find thoughts keep arising and you will no doubt get caught up in them (that's normal). When you recognise the thoughts arising, just use that as a trigger to come back to the mantra. Have a listen to Michaels CD's and see how you get on first, then you can ask specific questions and we can help.
As for meditation position, there's no real need to sit cross legged or in the lotus position unless you find that comfortable yourself. Lying down is often discourages as that acts as an indicator to the body that you are going to sleep and often that will happen. Just find yourself somewhere comfortable to sit where you back is supported upright and the head is not supported (if you start to fall asleep the head will start to fall and this will bring you back). The straightness of the back should be such that the spine supports the body without it falling over and the head position is resting on top of the shoulders/spine without any need to forcefully hold it there; i.e. you could literally sit on the edge of a chair and let the body relax in an upright position without any support. To start with though, it's ok to support the back and you can try other positions later on or as you wish till you find what's right for you.
Whatever happens, even if you fall asleep or keep finding your mind wandering, just remember that this is normal and it will become easier with practice. When I first started out with meditation I can remember managing to keep my focus on the mantra for perhaps 30 seconds to 1 minute before my mind brought up thoughts and it could then be several minutes before I recognised I was in a daydream and brought myself back to the mantra. It still happens now to some extent, but not so much, thanks to consistent practice.
Also, consider what time of day you are meditating? If it's last thing at night, you are naturally tired, but if it's first thing in the morning, providing you've had a good sleep, you may wake up with the mind telling you you are still sleepy, but if you meditate first thing, you'll come out of the meditation feeling more alert and awake and ready to get on with the day. Of course different people have to fit in meditation to suit their own lives, so that's just a little guidance on things to consider.
Keep us posted how you get on.
Hugs
Giles