Strange so it sounds its not connected in any way with your PC?... then wherever you plug the PC in the channels stop?....hmmm only thing that would strike me mate is your ariel must be running close to your electricity plug point wires and the computer is interfering... does your ariel use a booster box to get the signal normally?
do you have any video senders or similar equipment attached to your PC?
TV card using same ariel cable?
Freeview channels? Freeview signals are transmitted on six clusters of channels, called multiplexes. If you find you're missing a few channels, or you can't receive some of the Freeview channels you're after, it could be that you're not able to receive one or more of these multiplexes. Each multiplex contains a collection of channels and multiplexes that are transmitted on different frequencies, and at different powers. For a list of multiplexes,
http://www.digitalspy.co.uk/terrestrial/mux/.
To see if you should be able to receive all multiplexes, put in your postcode at the DTG site.
http://www.dtg.org.uk/consumer/coverage.html
Normal reasons for not being able to receive a multiplex are:
Your aerial was designed to work on a specific frequency 'band' , and that band excludes the multiplex frequency (see below).
You're getting a weak signal, or interference (see below) - Normally the best advice is to consider an aerial upgrade, but in some cases, trying to reduce interference or using a signal booster may help to pull in signals for a weak multiplex. Note that if there's a weak or noisy signal, a booster won't see enough signal to boost.
The transmitter running the multiplex could be transmitting on a lower power (check for engineering work)
Your aerial just isn't up to the job (perhaps it's a indoor/set-top aerial?).
Here are a few bits of advice on improving your signal:
Upgrade your aerial - To get good pictures, you need a good quality external (roof) aerial. An aerial upgrade is always a good bet for getting the best signal possible. Seek professional advice and get the best that you can afford. More details can be found elsewhere on this page.
Boost your signal - If you're in an area of weak signal, consider a signal booster. For best results, consider a masthead amplifier. Note that mains-powered boosters are only really effective in some circumstances (to boost quality, not signal) - if the signal is poor, a booster may only amplify the noise.
Check splitters - If you're splitting aerial cables so that you can get TV in other rooms, consider a mains-powered booster with multiple outputs. This can help to overcome loss in the cables.
Cabling - Sorting out your aerial cabling can make a serious difference. Just replacing your flylead from the aerial socket on your wall to the Freeview box can make a difference. Use a high-quality co-ax aerial cable with gold-plated co-ax connectors (from Maplin), don't run you aerial cable anywhere near your mains cables, and don't use longer cables than you actually need. Another option to try (although it didn't work for us), is to wind your aerial flylead through a 'ferrite ring'. These are designed to cure electromagnetic and RF interference at the source. Available for about a pound from Maplin Electronics.
Another annoyance experienced by some users of Freeview, is electrical interference, such as picture breakups when a light is switched on, the washing machine is on, or there's other electrical activity. Digital Terrestrial TV is prone to electrical interference of this kind, and here are a few hints to help you cure this:
Keep your aerial leads and connectors as far away as possible from electrical mains leads
Make sure you're using good quality leads to connect from your aerial to your STB, and from your STB to your TV or video. For a new extra pounds, a good quality aerial flylead cable with a gold-plated connector from Maplin Electronics may help.
Joined cables and lots of unshielded adapters provide a weak link in your setup - make as few joins as possible, and keep joins away from all mains cables.
If this doesn't work, a good quality external roof aerial with good quality unbroken cable straight to your set top box should be the answer.
hope this might help!!
its shit loads better freeview is bleak now in comparison 
