I currently keep my old HDs that are above 10GB. All my Desktops have at least one Mobile tray and with subscriptions to MSDN, and TechNet, I have plenty of software to test, and experiment with.
So, as a frame of reference.
Win 7 Starter on a 939 Athlon 64 3200+, 2GB RAM, HD 10GB.
OS + Firefox + Word + few small applications. Hiber Off, and pagefile set to 1GB static.
It works well and there is about 1 GB empty room on the HD.
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Installation of Win 7 Ultimate/Enterprise/Pro with full Office 2007 + Nero +Paint shop Pro + Sony DVD Architect, and few other application of this caliber (No Games beside the one included with Win 7).[/FONT]
[FONT="]It takes about 20GB, works well on a 30GB HD.[/FONT]
[FONT="]However if One wants to leave room for future in place type of Upgrade more head room need to be available. Thus 40GB (without games) is the Entry Level HD that is needed as the primary OS HD for Win 7.[/FONT]
[FONT="]If you have Games and other space hogging application, check how much it takes and add it to the minimum. If you have more RAM it adds to space added to the pagefile and Hiber and need to be taken into consideration too.[/FONT]
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Totally Rock Bottom. Toshiba laptop 2GHz Celeron, 512 RAM, and 30GB HD.
Win 7 Ultimate RC1 works on it. It can be used for surfing Word processing and streaming Music and video. It is not fast but as long as it Not multitasking it works OK.