No the mpcache file is not a EXE but a Bin file and can be quite large.
It has been suggested that my computer might be infected as nobody as yet seems to recognise this mpcache bin file.
In Windows Explorer it appears under Microsoft Antimalware folder inside the scan folder.
My computer however runs ok and I used both MSE antivirus and Malwarebytes and no problems have been reported with any scans todate. I did have McAfee Security Centre installed about a year ago but did not used their dedicated tool to uninstall and this has been suggested as a likely cause of the mpcache bin file but if it is it has taken a long time to appear?
This mpcache file has only recently started to appear when using Piriform Defrag but not everytime. When it is present it does not defrag at first but does when highlighted seperately.
I was having this same problem, only it wouldn't defrag for me even when I highlighted it. It will defrag with a boot-time derfrag. This might have already been mentioned elsewhere, but I actually couldn't defrag the page file at one time either, but I discovered a little trick with that. If you go into your settings (control panel/system/advanced/performance-settings/advanced) in XP and set your paging foe to 0 (no paging file), you can defrag everything, then just set it back up to whatever you need it at. It will start fragmenting soon after, but at least you'll be starting with a clean slate.