for Svchost.exe problem
may try this webpage
anyway, i havenot tried by now

http://www.trendmicro.com/vinfo/virusencyclo/default5.asp?VName=WORM_NACHI.B

or maybe try d/l "fixwelch" from symantec,but this tool isnot perfect either,canot delete forever(my experience)
http://securityresponse.symantec.com/avcenter/tools.list.html
W32.Welchia.Worm

or try d/l "stinger",also not perfect one
http://vil.nai.com/vil/stinger/

just try and look for the better method ;)