Q: really slow mac

My iMac os 10.7.5. has been running so so slow.  Both internet and internal programmes.  so went to disk utilities and performed verify disk and all was ok.  I then noticed four dmg s with installer underneath.  I cannot find out what these are exept they are in a temp folder.  When I go to eject or move to trash message box says   "the disk "BYeuUz.dmg" could not be unmounted.   Make sure all applications and files are closed on this disk"     There are four more of these files, they have changed as well.  Does anyone have this problem and could this be the reason for slowness