You should be fine as long as...
1) You NEVER do this using company resources. You'd be shocked at how many folks use company PCs, phone numbers, mobile devices, email addresses, etc. for stuff.
2) You ensure that you cannot be snagged doing this on company time. There's no reason that anyone else has to think this is anything other than a nights / weekends deal
3) You don't tell anyone at work, even those you trust. If they don't know, they don't have to lie.
4) Check your company handbook to see if a scenario like this is addressed. If it's not, you're cool, but still follow 1-3. If it is, it's not necessarily a fireable offense (unless overtly stated) but it is something that will readily be used against you if someone is already gunning for your ass.
