I have a little Acer Revo without a CD-ROM. External CD-ROM is expensive, so I reinstalled my little PC from USB stick.
First, you have to have a stick not smaller than installation CD/DVD of Windows 7 or 8. Next, motherboard must support Boot from USB so you can boot to setup.
The procedure goes like this, because we must prepare the USB with DISKPART. In my case CD/DVD-ROM is D: drive and USB flash drive is E: drive
SELECT DISK 1
CLEAN
CREATE PARTITION PRIMARY
SELECT PARTITION
ACTIVE
FORMAT FS=NTFS
ASSIGN
EXIT
Next, make USB bootable
Enter to your CD-ROM and subfolder boot
D:
CD BOOT
BOOTSECT.EXE /nt60 E:
And then copy all from CD/DVD to your USB. I used xcopy, but you can copy it from Windows explorer.
XCOPY * E:\ /E /Y 
Plug in you USB into a computer, and boot from USB.
Good Luck
