When I connect ten computers over a 100 meters (328ft) UTP Cat5, is the signal loss so large that it will affect my network performance? These computers perform simple office tasks, and the 100m utp is a temporary solution.
UTP cable does not have a maximum length.
However, the protocol that runs over UTP has a maximum length for performance and technical reasons (such as collision detection in Ethernet).
You're probably talking about Ethernet, and the maximum end-to-end cable length in Ethernet (at least for 100Base-TX, 1000Base-T and 10GBase-T) is 100M.
Assuming you're running full-duplex, you're withing spec (and hence should be fine) as long as no switch-switch or switch-host connection exceeds 100M.
Maximum cable length in Ethernet networks is 100M; if you're really in need of a connection extending any further, you should place a simple hub/switch between two shorter cables and it will happily resend the signal to the far end.