You shouldn't need to scan each user account, but it wont hurt to start with.
What are you using as spyware scanner and firewall??
I suggest that you use Norton 2006 edition of firewall and Microsoft antispyware and when paranoia sets in use Spybot search and destroy and thenAdaware SE (personal edition). All the programmes are free (except Norton) and one picks up malware that others may 'pass over'.
Happy Christmas and happy PC health too!