Zendarin's link takes you to an excellent How To: on mounting windows file shares on the command line, meaning things you type into a Terminal window. Here's how I get it to work using the Dapper graphical desktop.
First, let's call the computer you want to connect to (from Ubuntu) the "target computer."
Make sure the target computer has Windows file sharing turned on. On Win 98 just right-click on a folder and select Sharing. On Win XP it's trickier, but the directions are in the help files. On Mac OS X it's in System Preferences - Sharing, turn on Windows Sharing. (Also, just for grins, make sure the target computer is turned on, booted up, and connected to the network. I know, I know, but these things happen...)
OK, now let's find out the name and address of the target computer.
Select Places > Connect to Server...
Click Browse Network
Wait a few... A window will open. Wait some more... A Windows Network icon will appear in the window. This is progress!
Double click the Windows Network icon.
It will open to reveal one or more workgroup icons. The icon name is the name of the workgroup. Write that name down, you're going to need it.
Double-click the windows workgroup icon.
Your Windows shares should now be visible. Any of them that are not password-protected on the target computer should be available just by double-clicking on the name of the share.
But what if the share is password protected, or double clicking the icon doesn't work? No fear. The simplest thing I've found to do is write down the name of the workgroup and the name of the target computer, then close the File Browser window and start over. It's fast, it's easy and it works. Here we go:
Places > Connect to Server...
Change Service type to: Windows share
Type the target computer's name next to Server. Being case-sensitive won't hurt, but it doesn't seem to matter. (This seems to be a required step.)
Type the share name next to Shares: (This seems to be optional, but being specific might be useful for getting directly to the share you want to access.)
Type a folder name, if you wish, next to Folder. (Again, apparently optional. You might want to do this if the folder were one where your user name matters for permission reasons.)
For User Name type the name of the user account by which you want to log in to the target computer. (Optional, unless the target computer requires it.)
For Domain Name enter the Windows Workgroup name. (This is required information.)
Next to Name to use for connection: type the name you want this share to be labeled in your Ubuntu filesystem. (Optional. Ubuntu will give it a meaningful name if you don't.)
Click Connect
A folder will appear on your Ubuntu desktop with the name you are using for the connection.
Double-click the folder on your desktop.
If all goes well, you'll see a message that Ubuntu is opening the share. Wait a few... A window will open. Wait some more... If asked, enter the password for the user account that you are logging into on the target computer. Wait a bit longer...
And there it is!
I have tried leaving various of the fields blank. The ones that matter most are the Server (target computer) name and the Domain (Windows Workgroup) name.
Frankly, once you make it work this way, you will recognize the bits that you need to type using Zendarin's method. As you gain experience, you'll find that the Terminal (a.k.a. Command Line) way is faster. It is the true Unix Way. But the graphical desktop is nice for taking it step by step. There is more than one way to do things in Ubuntu, and that's a good thing.
Iowa Dave