I recently got FO3 GOTY for the PC and have decided to see how to install mods. I want to start with danthegeeks underground hideout mod. What do I have to install to download this mod? I have archive invalidation, and tried to download Fallout script extender and Fallout mod manager and got errors spit at me. Do I need all these? Does anyone know what's wrong?

The FOMM error get is: Unable to get write permissions for fallout's instillation directory. Please read the "Readme - fomm.txt" file found in the fomm sufolder of your fomm installation. I read the readme and I might uninstall and reinstall somewhere else except in program files, but will this delete my save games?

And I think i am just doing something wrong with winrar in FOSE

Oh, and I have windows 7, if that clears anything up.

Sound like a ordinary case of UAC being a [censored]. You can solve this a few ways: Reinstall the game(and mods) outside the program files part of your main drive; set up UAC to allow FOSE and related things to edit the folder; or just turn off UAC.
First one can take the most effort or least, depending on how many mods you have installed, second is usually the hardest or second hardest, and third choice can cause problems with things running on your computer without your permission(not much of a problem if you know your computer and have anti-virus and other protection methods).

Problems to get past as follows:

1-Burn any machine using Winblows7. Heck even vista as sad as that is works better. Go figure.

2-If thats not an option....you should at least seriously think about it....no, really. lol

3-Ok joking aside =)
As YX mentions about UAC. If you have adequate protection on your machine, you can simply turn it off and not have most of the problem you will encounter even after the stuff you are encountering now.

4-Lets assume you uninstalled everything and will do this fresh. New game install with nothing saved etc etc. Also assuming UAC is off. You do NOT need Archive invalidated at all. First install the Mod Manager. Then install FOSE as it will integrate into Mod Manager. Then go into the mod manager and click the option for "Toggle Invalidation". Bam! Yer all set for invalidation. Occasionally only after a texture replacer mod is used will this be of use in the future. But normally its just paranoia and you are all set. Next up is to gl and click the "Install Tweaker" in Mod Manager. Click the "Use fake xlive dll" and apply, then close. Get the load order of you DLC's in correct order. Then you are ready to start modding.

If you cannot do any of those things then you will probably have more errors and will not be able to mod properly...there by getting on our nerves. Go to the bugs/error section there after with problems. As this is the modding area. Thank you =)