Outlook Contacts disappeared

One of my XP (Office 2003) users returned from vacation and found on the day after his return that his contacts had disappeared. In its place were some folder names that looked somewhat familiar but did not belong to him. His email is intact and I could locate his OUTLOOK.PST file. On the day of his return, he did a synchronize with his palm pilot. When we attempted to do a second synchronize, we encountered an error on OUTLOOK which indicated the possible presence of a virus and recommend to NOT continue. Any suggestions re: how to proceed? Thanks in advance.