Start-run-msconfig, startup tab, uncheck your "Gateway" if there. Otherwise, need more info.
Next time, defrag in Safe Mode. It bypasses startup files.
Also find Millenium (ME) Defrag.exe, smaller and faster.
With 18gb, you'd save yourself time and loss by having a 2nd partition, but it's a little hassle and needs Partition Magic ($$), or a spare Hdd for such. That way, your main C: could be lost and you could use-backup from D:...
Saves time on Defrag and Thorough Scan.