Basic Computer Virus Types:

Direct Effect viruses – Those are the standard type viruses that when executed, they replicate themselves in the current directory files and most probably the files under the PATH directory (being main windows files etc..). They could cause partial damage or no damage at all, but they still replicate continuously.
Memory Resident Viruses – Those viruses reside in memory and the problem is that they can interfere to many programs and operations taking place in order to corrupt and render files and programs unusable.
Viruses that delete or overwrite critical file information – Probably the worst kind you could encounter, these viruses most often overwrite a file’s important bytes with random sequenced bytes and make them totally unusable and cannot be recovered. At such viruses, many files tend to be destroyed once and for all and format is most times the only solution.
Macro Viruses – The most well known macro virus is of course the Melissa virus. Macros are in fact a known programming standard, meaning a series of instructions or commands used to execute operations that most often need time programming or need to run on special occasions. The most known way for a macro virus is through Microsoft Word. Microsoft has an embedded macro language and it’s possible that once a doc file gets opened, a virus gets launched. That is the reason why you should double check an email attachment sent as a doc or other macro type (and as an exe too of course).
Boot and FAT Viruses – These viruses affect the boot sector of a disk or floppy and can make it need a full format before it works again. A destroyed part of the boot sector means that the medium can no longer boot. The same applies with the FAT file system at which viruses launch direct attacks and make it unusable in parts.