What OS (Win XP/2000, etc) are you using? What type of anti-virus are you using? Have you performed any anti-spyware scans? What were you doing just before this error started?
A Runtime Error is an generic error message that occurs during the execution of a program and is usually accompanied by an error number, a "file not found" message or sometimes a "Do you wish to Debug? message. These type of errors can occur for a number of reasons to include hardware failure, program bugs, software application problems, file removal and malware. See What is a Runtime Error?
If your computer seems to be slow, read Slow Computer/Browser? Check here first; it may not be malware
. There are reasons for slowness besides malware - i.e. disk fragmentation, disk errors, corrupt system files, too many startup programs, unnecessary services running, not enough RAM, dirty hardware components, etc. As your system gets older it becomes filled with more files/programs and has a natural tendency to slow down so cleaning and regular maintenance is essential.