Problems can be caused by users deleting or modifying folders that are used by either the system or application software.
A computer infected by a virus will cause a range of problems from slowing down to data being removed or corrupted. These days it is essential for a computer attached to a network or the Internet to have constant virus protection. Infection can occur by inserting an infected floppy disk or opening an e-mail attachment that is infected. Most companies now have a mail sweeper to check for suspicious attachments and for spam e-mails which can mask viruses.
You should always have clean bootable media, such as a flash drive, with a virus scanner or a separate CD with one.
Although closing programs should release the memory for others to use, there are times when Windows does not recognise that the space is free and starts to use more of the virtual memory instead. Restarting the computer will reset all the memory.