The most common one BOZO, needs a macro to spread, and macros can be part of a Word template. Microsoft is distributing fixes free of charge for those viruses. Cannot access the hard drive, when booting from drive A. Files have strange characters in them. Even with a new battery, the CMOS settings get lost. A program size changes.
Programs take longer to load.
Strange messages appear on screen. This should only be of concern to you if you exchange diskettes with others or download programs from unknown sources from the Internet. A virus checking program is a good investment if you want to be completely safe. The two most common ones are Mcafee and Norton Antivirus. They are available from any software vendor.