I have Sony Vegas Pro 13 and I've been using it for a while with no problems.But recently,after I render a video there is a huge drop of quality.In the preview the quality is much better and I use bits from the same videos I used in the edits I did before (when the quality after rendering was good).
I don't understand what is it with this sudden loss of quality and any help would be greatly appreciated :)

Well, it's been a while since I used Sony Vegas Pro, but it depends on what kind of file you save it as and what your settings are when you render the video. I advice you to search on youtube for "how to render a video on sony vegas pro 13". I know there are a couple of videos that show how to. Hope they help

you might want to reset everything to default in the render menu

Try changing the settings around a bit for what it's rendered into, some formats hold their quality better than others.

Depending on what you're doing of course, if it's for Youtube you're going to have to stick to a slightly lower quality unless you want to spend a year uploading each video. If it's just for you and a smaller video .avi is the closest you'll get, but be warned, a RAW avi file can easily eat up tons of room on a harddrive. I personally use WMV for online uploads these days personally, least drop in quality for the filesize I want, but I always keep the files around incase I ever want to make a higher quality video.