Good news that you have not lost anything Ian.
The Operating System and applications are generally not worth backing up unless it's a system that is critical that it's operational 24/7, which I very much doubt is the case.
If so you only need to worry about the data (emails, documents, spreadsheets, accounts, etc) as the other stuff can be reloaded from the original DVDs. You can backup to USB memory stick or external hard drive, just depends how much data you have. You can copy the files across manually but automating the process with a backup application makes it simpler (e.g. A double click on an icon to backup, or scheduled to run at a specific time each day) and more foolproof. If you PM me I will suggest a good backup app that is FREE.
Don't just backup to one external drive or memory stick, get three and backup to them in rotation. And don't keep the backup media in the same location as the PC, take them offsite, as otherwise the fire / flood / burglary will destroy your backups as well as the original data.
If all that sounds like too much effort sign up for a reputable online automatic backup system and let that look after your data backups. Don't bother with the free or really cheap ones, they generally don't work properly when you try to restore from them (I have customers that have been there and done that). We are a reseller for Clunk Click backup from RISC Group and although it costs a few pounds a month it works really well' and your business data is stored in the UK, not in another country so you don't have any Data Protection issues. Your own IT provider will be able to recommend a good online backup system if you want to go down that route.
Glad it all worked out OK in the end, I know that thinking you have lost data is a stressful situation.
Rib Tigga (Avon SR4 Rescue)