Lets get down to business.... Like eaglestrike7339 said you can ping your Samba server, however the first question to ask regarding your samba server is if have you assigned a STATIC IP?
Here is how I would set up Samba (not to sure where Ubuntu 7.04 has it in the GUI interface but you should find it under the menu options in something like server or admin >> samba).
1) open the Samba GUI interface. (maybe; actions >> server >> samba?)
2) create a samba user (there are a few settings to worry about with use permissions but we can worry about them once your share is visible on the network)
3) set your samba server work-group to the same work-group as your windows network (just makes it easier for the whole windows network to see it, you shouldn't have to do this but i would recommend it)
4) create a share... so you make a share /home/(user)/samba_share. make it both visible and read/write-able (that is if you want it to be write-able)
5) open a terminal... su - root.... type: service smb restart, make sure it everything starts ok. if it does then type: chckconfig on, this will start samba every time your server starts or restarts.
6) go to one of your windows machines and see if you can see the new server under: my network places >> view network connections (I'm assuming your using XP). if you can not see it still then click tools, go to folder options and then the second tab... check to see if search for networks connections is on, if not turn it on (this looks for network shared printers, shares, etc. you dont need this on but you can turn it on)
7) if you still can't see the share let me know. at this point i would again ping your server and see if you get a result. if you do try mapping a network drive.... post back if you get back to this point and still have problems. See this link How to Map a Network Drive