No. To install application, find a .deb file for it.
http://maemo.org/packages/

Make sure that you choose correct operating system (Fremantle) and correct architecture (armel or ix86 or something like that).

After downloading .deb to N900, disconnect N900 from PC, find .deb in File Manager, click it, Application Manager will try to install it. If it doesn't say "successfully installed", go to Menu->Log. Here you can see one or both of the reasons:
1. "Failed: ..." Some additional package is needed, download it, too.
2. "user/FOO" is needed. This package has to be installed in command line (X Terminal).

Command line: you have to be very careful here.
1. Install rootsh (there will be no problems, I suppose);
2. Open X Terminal;
3. Type "sudo gainroot" (without quotes) and hit Enter;
4. From now on you have authority to break system down by incorrect commands; don't forget it, and don't type random commands, and don't allow a cat to use the keyboard;
5. Type "dpkg -i PATHTOTHEFILE" (without quotes) and hit Enter;
6. If the path is correct, it will say either ".......Setting up THEPACKAGE", and it means success, or ".....Error were encountered while processing THEPACKAGE", and it means that either you need more packages to be installed (it mentions which) or that the deb is incorrect;
7. If the path is incorrect, it will say something like "... not found"
8. As soon as you have finished installing what you wanted, or made sure it's impossible to install it, type "exit" (without quotes) and hit Enter (thus you exit dangerous role of root), type "exit" (without quotes) and hit Enter (thus you close Terminal window completely).

Some useful hints: in browser you can see full path correctly. Like, folder Documents in file manager is "/home/user/MyDocs/.documents" in X Terminal and "file:///home/user/MyDocs/.documents" in browser.

If you want to know more about command xyz, type xyz --help. Like: "dpkg --help".
If you want to see more commands, hit "Tab" button twice.
Don't hesitate to ask questions and to search the Wiki
http://wiki.maemo.org

P.S. I don't have Internet on mobile, as I haven't learned to make a home "Ad hoc" link between PC and mobile. So the only connection with it is USB cable.
P.P.S. I'm going to try algorithm of rnb89, but I will need to log in as Administrator for it, so not immediately. I hope the algorithm is all the same on Windows XP.
P.P.P.S. I managed to make ad-hoc connection, but Internet sharing couldn't be turned on, because it would change IP address of PC to 198.68.0.1 or something like that, and this in turn would separate me from LAN and, most likely, Internet (and I'm a server on this LAN, sharing many useful files, so I cannot afford to destroy LAN just due to Microsoft's whim of changing my IP; so I will use USB cable).