My 5370C has always been temperamental like that.
I've found two workarounds - having tried the normal solutions including downloading the not too recent but huge drivers update from HP.
1. Reboot the computer whenever you want to use the scanner - after a couple of hours connected even with scanner plugged in to the power and connected by USB it invariably freezes.
2. Scan from Photoshop Elements or higher. The results are far better you have more scanning options & it is never once frozen. (I haven't tried the latest MS picture packages that come with Office 2003 they may be equally effective.)