Serious help needed with networking!

I'm currently in the process of updating my office computer and decided to do it myself. I put together all new PC's running Vista Business 64 bit edition and have the network running but it's not at all stable.

I have a business DSL connection with an older modem. The modem is connected to a wireless D-link router which is connected to a linksys 8 port switch.

From here the 8 port switch connects to a 24 port switch which distributes to network terminals (currently 4 PCs connected and 2 laser printers).

Connected to the 8 port switch I have another printer, a NAS server (Synology 207+) and a PC.

Here's where I run into problems... I can connect, and I can see all the connected hardware from some of the PC's but not from others. I can connect to the internet on all PC's, but it disconnects at times and I have to re-enter my DSL username and password whenever a PC disconnects. I can also see the NAS when I look at the network but when the Synology software searches it cannot detect the hardware.

The wireless works, but again I have to enter my DSL username and password.

On my home network, I can simply plug and play with any PC that's been given access. I don't need to type a password every time. That's what I would like to accomplish on my office network. I'd also like to be able to start putting files on my NAS and giving the appropriate users access.

Any help would be greatly appreciated.

Thanks