The Cheltenham forward was Hereford's leading goal scorer in their Conference years and his double strike on his return means that his second stay could well be longer than the initial one-month loan that has been agreed between the clubs.
County made two changes from their win against Torquay the previous week but Graham Turner decided on major surgery in the Bulls' line-up. The Hereford chief made five changes with returning former United players Ben Smith and Guinan both included from the start.
The new boys played their part in an entertaining first half, but Hereford had to deal with a setback when they fell behind in the sixth minute.
In County's first attack of the match, Matt Somner crossed from the left and Jason Lee's deftly-flicked header crept into Wayne Brown's net off the far post.
But the Bulls did not take long to get back on level terms from an excellently-created goal.
Guinan slipped a pass to Alan Connell who laid the ball off to Smith. His shot came back off a post bit only as far as Trent McClenahan who thumped the ball into the empty net.
After plenty of good attacking play, Hereford went ahead on 26 minutes. Smith played a first-time pass through to Guinan who took the ball around goalkeeper Kevin Pilkington before sliding the ball into the net.
Smith came close to adding a third two minutes later when he raced on to a pass from Rob Purdie but slid his shot across goal and inches wide.
Andy Parkinson should have levelled the scores when he clipped the ball wide from a great position after outwitting Simon Travis. But Hereford again threatened just before the interval when Smith, who was marginally offside, fired Purdie's pass against an upright.
County piled forward at the start of the second period and their pressure paid off when Lee notched his second goal with a fine near post header from Gary Silk's cross.
But Hereford, and Guinan, were not to be denied. Four minutes later, the on-loan striker raced on McClenahan's pass and fired his second of the game past Pilkington.
After that, although County pressed forward in numbers, the Bulls held on for their first win of the year.