Win XP Laptop can't see Mac OS X 5.8 & Vice Versa

A month ago, I switched from DSL with an Actiontek modem/router using WEP to a cable modem and an SMC Barricade router using WPA-PSK TKIP. Ever since the switch, 3 things have been happening:

1. The Mac (Intel-based running OS X 10.5.8) drops its connection OFTEN. Have to stop/start Airport to get it going again. Husband is NOT happy.
2. The Mac can't ping ANYTHING (operation not permitted) and it can't be pinged from my other system (PC with Win XP SP 3 Home).
3. I used to be able to share files and printers with no problems whatsoever. Now, I can connect to the laptop from the Mac ONCE IN A WHILE - it disconnects frequently. I can't connect to the Mac from the laptop - other than being able to see "Printers and Faxes" SOMETIMES when I use Start/Run and enter the Mac's IP address - up comes printers and faxes - but if I add to that the path to my desktop on the Mac - nope, nothing - can't find it.

I know I'm entering it correctly as that information hasn't changed from when I was on the DSL/Actiontek, other than the IP address. Both systems have the same workgroup name and I've made sure the subnet Mask is the same, etc. NO FIREWALLS are enabled on either system - using the router firewall.

I have spent more than 30 hours searching for a reason why this is happening. Right off the bat, I blamed the router since everything was hunky-dory with the old one. So, I changed the settings on the new one to match the old. Didn't help. I've been in contact with both the ISP and the router support folks and tried everything they have told me to do. Nope, nada, nix. I've read a bazillion posts in various forums, tried rebooting everything, resetting the router, reconfiguring a variety of settings on every device in the mix - on and on and on until I'm just burned out.

Now I'm beginning to wonder if there's something going on with the Mac - maybe a recent OS upgrade fouled something up that wasn't fouled before the switch to the new network - I just think that's too much of a coincidence - maybe swapping out the router for a new one (the ISP loans them for free) might help, so that's up next.

If a new router doesn't fix the problems, then I just don't know where to go from here. I'm hoping there's a major geek with some major help on this forum - I'll trying anything at this point. Thanks!