When Cameron boots up his computer the CD drive is mounted as F drive however after inserting the cd he goes to D drive to run the install program.
Although not written in law the officers who arrested the suspect Cameron Walker would then NOT be carrying out the search of his property under s18 of the Police and Criminal Evidence Act (PACE) 1984 due to cross contamination - making it highly likely anything found or seized to be inadmissible as evidence.
It is never explained how Cameron is able to pay for the very expensive upgrades to his 1990s PC. He becomes strung out on acid in order to immerse himself in the game, and so presumably is incapable of continuing in his job any longer. However, given that he needs money to fulfill his mission it's clear that he must have found some sort of work-life balance, just sitting behind his computer playing the game would have ruined everything.