Is there a reason that you are using Ubuntu 7.04? The current version is Ubuntu 8.10, and you might have a better experience with that. Among other things, a pendrive version of 8.10 will give you the possibility of making and storing changes. If you want Ubuntu and your hardware is somewhat limited, there is Xubuntu.
On what hardware are you running Ubuntu? Although you can download and easily install 8.10 to a pendrive, it may operate more slowly than an installation to a HDD. There is this link; is it what you used?
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