The Go programming language, 100 characters:
package main;func main(){n:=42;c:="x";for i:=2;n>1;i++{if n%i<1{n/=i;if(n<2){c=""};print(i,c);i--}}}
My program, with the correct indentation:
package main
func main() {
n := 42 // or whichever input number you like
c := "x" // separating printed integers
for i:=2 ; n>1; i++ {
if n%i<1 { // n%i==0
n /= i
if(n<2) { c = "" } // n==1
print(i, c)
i--
}
}
}