It's quite normal that there's a difference in image quality between your vidcam and the captured movie on your harddrive. There are many reasons, but these are the most important:
*Resolution (source <-> output)
Depending on your Vidcam type, this resolution can be something like 360 x 240 (VHS), 400 x 300 (S-VHS/Hi8), 700 x 600 (DV). (these aren't the exact values, but just to give you an idea).
Most TV-screens are only capable of VHS/S-VHS resolutions (higher resolutions like DV are "overkill" on those TV's). The newest TV's (HDTV) can show up to 1024x768 and more; the DV-tape and DVD neighbourhood. Off course, you can display higher resolution sources on a normal TV (but you won't see all the details provided by the source quality).
However, the other way around; displaying a lower resolution quality on a better output, will result in "stretched" resolutions (blocks and the (ugly) artefacts mentioned above). And this is what happens on your PC-monitor (which has a resolution of somewhere 1024x768).
- Capturing device
It's easy to understand, that you'll need something that can transfer your source quality to your PC in the best possible way. This means, when you have a high quality source, you'll need high-quality transfer (or you'll lose quality). This is actually the most important (and most budget-related) factor. Professionals use very expensive captureboards to capture analog or digital tapes (can be up to 2000$ and more; Canopus, Pinnacle DC1000/DC2000).
I don't think this is an option for the average family movie
There's light at the end of the tunnel though There are cheaper solutions (pfiew!) The cheapest option is:
For DV-tape source: any standard Firewire card will do; with an easy to use software package from Ulead or other packages, this is suitable for most home-usage.
For analog sources (VHS/S-VHS/Hi8): you'll need a PCTV-card that's capable of capturing through analog video-input. This card gives the best results in the resolution range of 300 x 240 (equals more or less the source)
I noticed loss in quality though (vague colors, dropped frames, smaller artefacts,..)
Hauppauge or Pinnacle PCTV are best known brands.
Inbetween the professional cards and the cheapest, there's the semi-professional hardware; like the Pinnacle DV500 or EditionDV.
Those cards are capable of both analog and digital (DV) capturing and have a hardware-capture chip (high quality compression) (whereas the PCTVcards use software compression). These semi-pro cards provide a very good capture quality (sharp and clear images, true colors, no artefacts) and are best used with software like Adobe Premiere.
I hope this cleared something out for you. If you need more info, feel free to ask (but give some more information about your setup please ; sourcetape, PC specs, budget you want to use,...)
If you want some tips on the capturing itself, editing the captured movies and saving the output (either back on tape or CD; with compression codecs and settings), just ask. (or read some articles on Doom9)