can see Mac on network but can't access it

Hello, I wasn't sure if I should post here or if it still came under the realm of 'Switcher Hangout', so apologies if I chose wrongly.

I have a new iMac as of yesterday and have been trying to set my network up. My network consists of a router wired to the iMac which has a Canon printer and an external hd connected to it and 2 laptops running XP (connected to router via wifi).

I seem (althought I don't know how) to have got it so I can see and access folders on my XP machines from my iMac but what I can't figure out is how I can access the folders and peripherals connected to the iMac from my XP machines.

I can see the iMac listed on my XP machines as being part of my network but when I double click it to access the folders etc it says it isn't accessible and I might not have permission to use it.

Could anyone help me? I am a complete Mac beginner so I'll need spoonfeeding I expect

Thanks

JJ