I have Win 7 Pro on a 3 yr old Dell Lattitude with 2GB RAM. No slowdowns. However, when replying in Outlook or editing an existing document in Word, the typed characters will suddenly start appearing further down the email thread or document. Never had this problem on the same machine when it was running XP. It;s damn irritating. I also don't like the file explorer in Win 7. Over all, things seem more cumbersome in Win 7 & Office 2007. Maybe I'm getting cranky in old age and refusing to learn new things.
I too see that on a 3 yr old Dell Inspiron 6400. My current thinking is that it's a trackpad issue. The trackpad seems to be more sensitive with the default Windows 7 drivers. Just having my wrist pressing on the palm rest immediately to the side seems to make the cursor jump.