Can I run another ISP on it (Don't want MSN) ?
According to the TigerDirect site, you can.
Can I use a USB Ethernet adaptor on it ?
Again, supposedly you can, but they didn't list which adapters were compatible.
I read that someone installed windows 98SE on it. Is that really possible ?
I don't really know, but it might be. It all depends on how and when the CF slot is accessed.
I really need it to access the internet through my existing LAN setup, which just requires the browser to be setup for LAN access. Is this possible ?
Should be. I've had no problem setting up other CE devices to run on my network.