Two part question:
Can WinXP screensavers be used in Win7?
Where can I find a copy of WinXP's Windows logon screensaver?
At least some WinXP screensavers work in Windows 7 (see this thread).
I would think that you would be best advised to copy the WinXP Windows logon screensaver (which I think is logon.scr) from a Windows XP machine, if you have one lying around, and then try it out.
Edited to add: in fact, someone has written a howto guide, so presumably it works -- although I wouldn't necessarily put too much faith in the download provided.