Never a good idea for a few reasons I won't reiterate here. You're taking the easy road with this one. IMO this makes you look like what they call a 'pizza tech' around here. A professional should support the client fully and do the whole job, not just fix the problem and move on leaving the client to fend for themselves after 30 days.
Customers appreciate being taken care of. Be a FULL SERVICE technician!
By full service, I mean:
- Blow out dust and clean it up a bit if needed.
- Test all hardware you can when you can - e.g. HDD scans, RAM testing, check for blown caps on the mobo, etc. to catch additional problems EARLY on.
- Remove the virus. During the process try to get an idea of where it came from (note for later.)
- Fix the damage done to Windows and/or other software.
- Update all A/V and web technologies that can be exploited (e.g. Java only if already installed, Flash, etc.)
- Test, test, test.
- Ensure the system is running optimal by performing any needed maintenance.
- Ensure the system has active A/V protection - don't leave this up to the client even with a 30-day trial. They could forget to purchase or worse - blame you if something happens, and especially they will wonder why you didn't sell them a full year.
- Remember where you think that virus came from? TELL THEM, and suggest what they can do to avoid it in the future if possible.
- Work with them on safe browsing habits.
- Ensure there is an opportunity for you to answer any questions they may have.
- Finally, hand them a nice invoice with ALL the work you did to the system above. Let them know you went the extra mile and the hard drive tests good. Something that reads more than simply "removed Win32/virus." Maybe even some pre-printed materials (or heck, even shortcuts on their desktop if they desire digital copies) with more information on safe browsing habits and the like.
You can even use all the work you just did and information you gathered as an upsell opportunity (e.g. more RAM if needed, you can offer a data backup service and/or upsell an external HDD for backups, etc.)
Heck if you REALLY want to impress, get a folder or manilla/plastic envelope 8.5x11, for each client with their name written on it and also your business card attached. Put their invoice in it along with the other materials you've given them. This can include not just papers, but anti-virus product CDs if it was a physical copy sold, data backups on DVD if you did that for them, etc. You can include a physical printed copy of the Windows/Office/etc. product keys even. Very useful in certain cases where a key is illegible on the case after time/rubbing/fading, or they lose their office disk case, or whatever. Tell them that in the future should they need to return for repair, to bring that folder.
Want more? Keep a DETAILED log of what you did to their computer - complete with specs and all. Then you're prepared to answer any future questions about past service. Documentation is crucial - so be specific. I've worked with too many techs in the past who just wrote "removed viruses" on the ticket/invoice and there is no detail.
I'm sorry to rant off-topic here, but the 30 day trial and other questions you've posted recently, combined with the fact that you've only had a few customers, I'm saying that it seems like you have a way to go and a lot to learn. Stick around and you'll pick up a lot of tips and tricks here and there.