H.264 conversion help please

i have some large H.264 HD files and some VC1 bluray files

My system is just short of handling these properly

i can play other HD files , just not these as mentioned

i see many Hidef files in avi/divx-Mpeg format that are said

to retain the resolution/quality of the original files they were

converted from Ie: H.264

Is it possible to do this without losing quality ? Because the

Large mpeg and .avi HD files i have seem very hi quality and

i`m told they are 1080i in many cases

Can someone please advise me on how to do this conversion

and programs to use , settings to be aware of etc ?

PS

i apologize if i hit the wrong subforum , but i cant see another

for this type of conversion

Cheers