Heya ecko,
My PC stats are much like yours, my FS 2004 folder is about 20 Gigs, and I have about 150 aircraft (both flyable and AI). My FS 2004 program takes about a minute to "fire up."
My first suggestion would be to disable some of the Windows XP processes by using a program by Ken Salter called FS Autostart v1.1 found here:
http://www.softpedia.com/get/Tweak/System-Tweak/FSAutoStart.shtmlNot to disagree with Matt, however, my second suggestion is that you don't really need File Management software with FS 2004 if you're using Windows XP. Merely do as I do and create a new folder in your FS 2004\Aircraft\ directory called Backup Aircraft. If you want to, you can then go to each aircraft folder (that you don't fly on a day-to-day basis) in your FS 2004\Aircraft\ directory, right click it, then select Send To --> Compressed (Zipped) Folder. This will create a zipped folder of the aircraft. Then all that you have to do is to Cut/Paste the new aircraft *.zip file into your newly created Backup Aircraft folder. You can then Delete the original aircraft folder from your FS 2004\Aircraft\ directory.
Since these *.zip folders are inside a Backup Aircraft sub-folder (and the fact that they're *.zip files too) FS 2004 won't read them when it starts up.
This will also help keep your hard disk size down.
Once you're ready to fly the aircraft, just extract the *.zip file and Cut/Paste it into your FS 2004\Aircraft\ directory (keep the *.zip file intact in your Backup Aircraft folder). When you finish flying, just Delete the aircraft folder.