Some units like HP had a not-so-stock install of the OS and required some patch before you installed SP1. Ask Dell if they had a similar issue for your model.
For reference -> http://www.theinquirer.net/?article=5624
Bob
Laptop-(Inspiron 2500) Sp1 went in apparently ok, then reboot got to scroll screen and no action for 10 minutes-safe mode will start-last known good won't, about to restore back to pre SP1-whats up with the no boot- searched a litle and find that I'm not alone...

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