Watch what you download and where from. If you insist on using a P2P program, please read File Sharing, otherwise known as Peer To Peer. (P2P)
It's important to always keep current with the latest security fixes from Microsoft. UPDATED WINDOWS
Usually one can have more than one anti-spyware, anti-malware type programs installed without issue, although one may decide to choose one resident real time protection.
The same does not apply to firewalls and anti-virus programs.
Rule of thumb is one firewall and one anti virus program installed to avoid conflicts, loss of program efficiency and system lock up due to both software products attempting to access the same files at the same time.
An Anti-Virus product is a necessity. It's a good idea to set these to receive automatic updates so you are always as fully protected as possible from the newest virus threats. Microsoft Security Essentials is also an option.
Happy safe computing!!