I just got hdtv from Comcast a couple of days ago. They connected the box to wall outlet and then coaxial cable to tv antenna input and those those y,pb,pbr cables from box to y,pb, pbr outlets on tv. This way, it's double connected so i have to switch to antenna to watch regular cables, or component 1 to watch high definition channel. I checked dct5100 manual and it looks to me that i should be able to watch all channels from one connection, but neither connection can display both set of channels properly. Any other way to connect it?

See pages 19, 20 of the DCT5100 user guide. Use picture in picture, it will help to see both outputs.

The manual that I have says that I only need to connect the y,pb, pbr cables to my tv, but that way I cannot watch the regular cable channels.

I use the component connection for viewing HD content, and use the RF Cable connection for viewing NTSC content.

I have the cable hooked to the 5100, then the component goes to the television, and the RF goes to the VCR. I have the RF out on the VCR connected to the television.

Make sure you set up the 4x3 over-ride correctly in the info settings, (power off, press info on the 5100).

My setup may not solve your problem, since I can watch the non-HD programming through the component connection just fine.

Are you trying to watch non-HD digital channels, or non-HD analog channels?

The correct way to connect the DCT5100 is as follows:

Cable drop to the coax input on the 5100
Coax out of the 5100 to the ANT (coax) input on your TV
YPbPr (component) out of the 5100 to component in on your TV

The DCT5100 has a built in 4:3 override function for non-HD signals. This allows for viewing all content, HD or SD, through the box and one input on your TV.

Now if you have an integrated set, some folks prefer to use a two-way splitter (-3.5dB signal loss) and have one ouput from the splitter to a 2nd ANT-in on the TV and the other line to the 5100 then out to the 1st ANT-in. You would then have to switch inputs on your TV to view analog channels on that 2nd ANT-in without going through the cable box. This usually provides better PQ on the analog channels (doesn't use the box to process the signal). Of course you could still watch all through the cable box without switching TV inputs this way.