Firstly I am not a Piriform employee. - These comments are one (experienced) user to an (alleged) other.
TBH it's much more likely that some enthusiastic IT tech has wiped the computers drive after you turned the computer in, to get it ready for the next user.
That's what is routinely done with company computers when an employee leaves, and there should be a log in the IT department if that's what happened.
(If they are not keeping logs for each piece of kit they handle then they are not doing their jobs properly).
Are they saying that it was definitely CCleaner that was used to wipe the drive, or are they saying 'something like' CCleaner?
(It would also be interesting to know how they think they can tell what was used to wipe a drive, after it had been wiped).
You might like to ask them what tool their IT department routinely uses to wipe drives when an employee leaves.
But you can't prove that you didn't do something, it's just not possible to prove a negative.
For example:
You don't need to provide an email to download and install CCleaner Free version.
You do an email to Purchase CCleaner Pro, but could have used a false email.
Both of those need to be installed, but there is an advanced CCleaner version that can be used without installing it.
So you can't prove that you didn't use it; but neither can they prove that you did.
More about Drive Wiping:
CCleaner's Drive Wiper is used to remove traces of <u><em>already deleted</em></u> files so that they cannot easily be recovered.
(Easily recovered - FBI, CIA, etc. may still be able to get at some of them with special forensic tools that are not normally available).
On a non-system drive it can also entirely wipe the drive, deleting everything on it so it can't be easily recovered.
For safety CCleaner's Drive Wiper cannot delete any files on a computers operating system drive, the one that holds Windows. It can only clear the free space on that drive.
So if there was only one drive in your PC then CCleaner's Drive Wiper could not have deleted any files from it.
(Other drive erasers may be able to fully wipe it, but in that case the computer would no longer contain Windows and so couldn't be booted).
In conclusion:
You need more details of just what they are alleging, and ask/tell them to prove their allegations.
"Innocent until proven guilty" remember.
If they think you did something wrong, then it's up to them to prove it.
PS. Just what are they alleging?
Any work related files should have been saved on a central server, or backed up to one at least daily, so they can't have lost any work related files.