Using a Company network gives the company the right to snoop the traffic and see what you are doing.
It also gives the company the right to dictate what you (the user) are allowed to do with that computer, which is their property.
With a single phone call, the employer can inform at any time they are able to make contact with you (the end user).. even 10pm on a Sunday... they might say please power off the computer immediately, do not press any keys; we have revoked your permission to access the laptop, you are no longer authorized to perform any activities on the laptop, and please return it to our offices at your earliest convenience, tomorrow morning
If you login to it after that point to grab a personal file, or delete some personal files, after that, then you would be committing the crime of unauthorized access to a computer system. So your access to "personal" files could be lost at any point in time, outside your control.
Likewise.... if they hand you a Linux system and tell you that installing software, or changing the operating system is not allowed, and you go format and install Windows 7 on it, or boot it from a Knoppix CD / Boot and Nuke CD / reimage it, or gain access to restricted accounts such as local Administrator (through hacking) to install unapproved software such as a file shredder, then you might be looking at civil or criminal charges for gaining the unauthorized access or the destruction of "personal files".