Any voting method is vulnerable to tampering if the elections officials who control the process are dishonest. On the old lever machines, the counters could be reset or not reset as needed to change the results. On the optical systems, unmarked ballots could be marked or marked ballots could be marked further to force their rejection due to "over-voting". And we all saw what a mess could be made of the punch cards. It is impossible to create a perfect method.
In a way, I think the electronics may be a little less vulnerable to "casual" vote fraud due to the specialized programming knowledge needed. There is a smaller pool of individuals who have the capabilities so there is less chance of there being dishonest individuals among them. In addition, the more involved process of needing to write the virus ahead of time, get access to the machines to install it, and finally restore the machine to normal so the fraud is not detected and the votes thrown out raises the chances that anyone trying it would be caught. Fraud on the older methods could be carried out by almost any individual, on the spot, and relatively little chance of being detected after the fact.
Personally, I think all the money spent on these new machines is a waste and was an over-reaction. The supposed systemic problem of vote fraud was an invention of people trying to affect the outcome of an election with little evidence. When there is real singificant vote fraud, there will be evidence. Some of the past isolated incidents that come to mind are precincts with more votes cast than registered voters, "dead" voters, absentee ballots mailed out that were already marked, etc. In all these cases, they should be fully investigated, the affected votes nullified, the individuals involved thrown in jail, and if necessary hold another vote with increased supervision.
However, if politicians insist that we spend money on these new systems, I do think the idea of a printed reciept is a good one. Not that it would prevent fraud, somebody will always come up with a way, but because it will make the individual voter more confidant in his own vote. But some will still suspect a rigged system any time they lose an election.