here I go

i have mpeg video files (coded from avi), and I'd like to use them to create a SVCD.. but
when in Nero I choose the format of the video if I choose PAL my videos are stretched, if I choose NTSC nero cuts my video (I can't see the subtitles, and I would need them cause I don't understand a word of japanese :P)

what can I do?

Put that mpeg-stuff to TMPGenc
Make your settings for svcd
Convert
Burn

but my files are already converted with that program, is nero that burns them bad

You're not converting them correctly in TMPGenc. My guess is that you have a weird resolution to start with and you are not converting it with the right settings in TMPGenc. NTSC SVCD requires a resolution of 480x480 and PAL SVCD is 480x576.

actually the original resolution of the video is 630x360... isn't there any way to keep this resolution making the svcd?

It is possible to keep the aspect ratio, but not the resolution. Your resolution is invalid for SVCD. You might want to consider make XVCD instead. Use a VCD resolution of 352x240 NTSC or 352x288 PAL, but use higher bit rates for the video than VCD allows for. You are going to have to convert your x360 resolution up to 480 (bad) or 576 (worse) depending on whether you want NTSC or PAL SVCD. You will lose quality by doing that kind of upconversion. You're probably better off to make a XVCD by going down to a valid resolution instead of trying to upconvert 360.