Usually that would imply another program is using it. Make sure you close ALL other programs (Including this page, as that means Internet Explorer is running) and most definitely mIRC. Close all the programs in the taskbar too, including MSN Messenger etc.
If you still can't delete it, scan your system with another antivirus. And another. And another. Don't just scan that file - scan your entire system. Use both virus scanners AND trojan scanners.
You may want to run your computer in Safe Mode. I don't know what version of Windows you're using, so see
this page for a number of results.
I've seen the files "Lass" and "LSASS" been used together quite frequently. LSASS is what is attacked by the Sasser worm. I suggest you get your computer scanned and find more information about Sasser
here aswell as make sure you have all the latest Windows Updates at
www.windowsupdate.com.
Regards,
