... Can I just migrate the pictures?
Yes. Download Windows Migration Assistant on your Dell: http://support.apple.com/kb/DL1557
You can safely migrate everything it will offer to migrate, including your emails, Outlook calendars, contacts, iTunes music, etc. If you have iTunes purchased music on your Dell then "deauthorize" it before committing it to the trash heap.
Any viruses that may have affected your PC cannot affect OS X.