If you have a bootable XP CD, you can recover that missing file using the Recovery Console [along with the EXPAND command].
I'm sure you can find lots of tutorials, using google, to help you get into recovery console and on how to use the EXPAND command.
this link tells you how to get into recovery console >>LINK<<
Once in recovery console, and at the C:\ prompt, you will need to switch back to your cd/dvd drive letter [assuming X: here], then, cd to your i386 folder
then, issue this command to expand the svchost file into the correct place.
expand svchost.ex_ C:\Windows\system32\svchost.exe
Confirm by pressing Y for yes. Remove the CD.
Then, reboot into the OS with safemode ....goto Start>Run and issue the command "sfc /scannow" (without the quotes)..reinsert your XP cd and let it scan and replace whatever corrupted/missing files it finds.
Hopefully this will allow you to get an antivirus installed to let you scan/clean up your OS.
Otherwise, you will need to use salem