Wireless Networking / Sharing / Web Access Trouble

I have a very confusing problem with a wireless network. Here's a snapshot
of my wireless network :

1. it's Ad-Hoc - between a main PC (connected by broadband to internet) & a
laptop
2. both are running XP Pro SP2
3. signal strength shown as EXCELLENT on both computers
4. sharing is enabled on both computers
5. I can ping both ways - very fast times
6. main PC is set to 192.168.0.1 and laptop to 192.16.0.2 - subnet mask 255
etc
7. they're both on the same workgroup
8. main PC CAN read and access the laptop files but laptop CANNOT read or
access main pc or connect to internet / Messenger. I get an error about
permissions!
9. both have firewalls and it makes no difference when both firewalls are
temporarily disabled.

Until a recent hard disk drive failure on the main PC, the setup worked fine
whereby both could be connected to internet, see each other's files and share
resources (eg printer) - all over a wireless network.

I don't know what's going on anymore. PLEASE can someone help me as I'm now
losing the will to live!!!

Thanks

Jan