On my network at home, I use static IP addressing - the following images may explain it a bit better... (I assume you are using WinXP on both PCs)
On both PCs, go to control panel and then network connections. Right click the network connection used for the router and go to properties. A box like this will appear...
Go to TCP/IP and then into properties... a box like this will come up...
you will find (because you are using DHCP) that it will have the option "
Obtain an IP address automatically" selected. If I remember correctly, you should still have the button "Advanced" available to click. If so, choose this and you will get a box like the following one.
Select the WINS tab and choose "Enable NetBIOS over TCP/IP" and click OK on all boxes to back out. Try again. As the PCs were working using a crossover cable, I assume they are both on the same workgroup?
Hope this helps...
Like I said above, I use static IP addressing rather than DHCP, so I always know the IP of each PC on the network (I have 5 including my laptop which is wireless) - purely as I'm too lazy to do an ipconfig on a PC when I work on it
Any problems, post back and I'll try to help some more